Assemble app version with pure Go Git and SemVer libraries

Previously the version of the application was determined by calling
`git` commands in a new shell process. This works in most cases,
but might fail if Git is not installed on the running system.
To prevent further enlargement of the required development environment
setup dependencies by adding more checks for external dependencies,
the `go-git` library v4 [1] (`github.com/src-d/go-git/v4`) has been
added:
"A highly extensible Git implementation in pure Go"

It allows to interact with the project repository and extrac required
information like the latest tag and commit of the current branch to
assemble the application version. To simplify the processing and parsing
of the version, the `semver` library v3 [2]
(`github.com/Masterminds/semver/v3`) has also been added.

The new `getAppVersionFromGit()` function assembles the version of the
application from the metadata of the Git repository.
It searches for the latest "SemVer" [3] compatible version tag in the
current branch and falls back to the default version from the
application configuration if none is found.
If at least one tag is found but it is not the latest commit of the
current branch, the build metadata will be appended, consisting of the
amount of commits ahead and the shortened reference hash (8 digits) of
the latest commit from the current branch.
The function is a early implementation of the Git `describe` command
because support in `go-git` [1] has not been implemented yet. See the
full compatibility comparison documentation with Git [4] as well as the
proposed Git `describe` command implementation [5] for more details.

[1]: https://github.com/src-d/go-git/v4
[2]: https://github.com/Masterminds/semver/v3
[3]: https://semver.org
[4]: https://github.com/src-d/go-git/blob/master/COMPATIBILITY.md
[5]: src-d/go-git#816

Epic GH-33
GH-92

go.mod

@@ -3,6 +3,7 @@ module github.com/arcticicestudio/snowsaw
 go 1.13
 
 require (
+	github.com/Masterminds/semver/v3 v3.0.1
 	github.com/fatih/color v1.7.0
 	github.com/imdario/mergo v0.3.8
 	github.com/magefile/mage v1.9.0
@@ -13,5 +14,6 @@ require (
 	github.com/spf13/cobra v0.0.5
 	github.com/spf13/pflag v1.0.5 // indirect
 	golang.org/x/sys v0.0.0-20191010194322-b09406accb47 // indirect
+	gopkg.in/src-d/go-git.v4 v4.13.1
 	gopkg.in/yaml.v3 v3.0.0-20191010095647-fc94e3f71652
 )

go.sum

@@ -1,19 +1,48 @@
 github.com/BurntSushi/toml v0.3.1/go.mod h1:xHWCNGjB5oqiDr8zfno3MHue2Ht5sIBksp03qcyfWMU=
+github.com/Masterminds/semver/v3 v3.0.1 h1:2kKm5lb7dKVrt5TYUiAavE6oFc1cFT0057UVGT+JqLk=
+github.com/Masterminds/semver/v3 v3.0.1/go.mod h1:VPu/7SZ7ePZ3QOrcuXROw5FAcLl4a0cBrbBpGY/8hQs=
+github.com/alcortesm/tgz v0.0.0-20161220082320-9c5fe88206d7 h1:uSoVVbwJiQipAclBbw+8quDsfcvFjOpI5iCf4p/cqCs=
+github.com/alcortesm/tgz v0.0.0-20161220082320-9c5fe88206d7/go.mod h1:6zEj6s6u/ghQa61ZWa/C2Aw3RkjiTBOix7dkqa1VLIs=
+github.com/anmitsu/go-shlex v0.0.0-20161002113705-648efa622239 h1:kFOfPq6dUM1hTo4JG6LR5AXSUEsOjtdm0kw0FtQtMJA=
+github.com/anmitsu/go-shlex v0.0.0-20161002113705-648efa622239/go.mod h1:2FmKhYUyUczH0OGQWaF5ceTx0UBShxjsH6f8oGKYe2c=
 github.com/armon/consul-api v0.0.0-20180202201655-eb2c6b5be1b6/go.mod h1:grANhF5doyWs3UAsr3K4I6qtAmlQcZDesFNEHPZAzj8=
+github.com/armon/go-socks5 v0.0.0-20160902184237-e75332964ef5 h1:0CwZNZbxp69SHPdPJAN/hZIm0C4OItdklCFmMRWYpio=
+github.com/armon/go-socks5 v0.0.0-20160902184237-e75332964ef5/go.mod h1:wHh0iHkYZB8zMSxRWpUBQtwG5a7fFgvEO+odwuTv2gs=
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+github.com/kr/pretty v0.1.0 h1:L/CwN0zerZDmRFUapSPitk6f+Q3+0za1rQkzVuMiMFI=
+github.com/kr/pretty v0.1.0/go.mod h1:dAy3ld7l9f0ibDNOQOHHMYYIIbhfbHSm3C4ZsoJORNo=
+github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
+github.com/kr/pty v1.1.8/go.mod h1:O1sed60cT9XZ5uDucP5qwvh+TE3NnUj51EiZO/lmSfw=
+github.com/kr/text v0.1.0 h1:45sCR5RtlFHMR4UwH9sdQ5TC8v0qDQCHnXt+kaKSTVE=
+github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
 github.com/magefile/mage v1.9.0 h1:t3AU2wNwehMCW97vuqQLtw6puppWXHO+O2MHo5a50XE=
 github.com/magefile/mage v1.9.0/go.mod h1:z5UZb/iS3GoOSn0JgWuiw7dxlurVYTu+/jHXqQg881A=
 github.com/magiconair/properties v1.8.0/go.mod h1:PppfXfuXeibc/6YijjN8zIbojt8czPbwD3XqdrwzmxQ=
@@ -27,10 +56,15 @@ github.com/mitchellh/go-homedir v1.1.0 h1:lukF9ziXFxDFPkA1vsr5zpc1XuPDn/wFntq5mG
 github.com/mitchellh/go-homedir v1.1.0/go.mod h1:SfyaCUpYCn1Vlf4IUYiD9fPX4A5wJrkLzIz1N1q0pr0=
 github.com/mitchellh/mapstructure v1.1.2 h1:fmNYVwqnSfB9mZU6OS2O6GsXM+wcskZDuKQzvN1EDeE=
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+github.com/pelletier/go-buffruneio v0.2.0/go.mod h1:JkE26KsDizTr40EUHkXVtNPvgGtbSNq5BcowyYOWdKo=
 github.com/pelletier/go-toml v1.2.0/go.mod h1:5z9KED0ma1S8pY6P1sdut58dfprrGBbd/94hg7ilaic=
+github.com/pkg/errors v0.8.1 h1:iURUrRGxPUNPdy5/HRSm+Yj6okJ6UtLINN0Q9M4+h3I=
+github.com/pkg/errors v0.8.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/russross/blackfriday v1.5.2/go.mod h1:JO/DiYxRf+HjHt06OyowR9PTA263kcR/rfWxYHBV53g=
+github.com/sergi/go-diff v1.0.0 h1:Kpca3qRNrduNnOQeazBd0ysaKrUJiIuISHxogkT9RPQ=
+github.com/sergi/go-diff v1.0.0/go.mod h1:0CfEIISq7TuYL3j771MWULgwwjU+GofnZX9QAmXWZgo=
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@@ -41,20 +75,55 @@ github.com/spf13/pflag v1.0.3/go.mod h1:DYY7MBk1bdzusC3SYhjObp+wFpr4gzcvqqNjLnIn
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