Move to go module, fix cli package reference

[amake]

Pull requests #50 and #52 opened up a can of worms:

1. You are now using github.com/urfave/cli v2.0.0 but this is not reflected in the glide files, so for instance I have a hard time updating the MacPorts port because it's no longer clear what your dependencies are pinned to

2. Trying to update the glide files fails because, as best I can tell, some dependencies have moved to the modern go module system, and glide can't understand it:

   % glide cc
   [INFO]	Glide cache has been cleared.
   % glide up
   [INFO]	Downloading dependencies. Please wait...
   [INFO]	--> Fetching gopkg.in/yaml.v2
   [INFO]	--> Fetching github.com/codegangsta/cli
   [INFO]	--> Fetching github.com/mattn/go-sqlite3
   [INFO]	--> Fetching golang.org/x/net
   [INFO]	--> Fetching github.com/andybalholm/cascadia
   [INFO]	Resolving imports
   [INFO]	--> Fetching github.com/urfave/cli
   [INFO]	--> Fetching github.com/cpuguy83/go-md2man
   [ERROR]	Error scanning github.com/cpuguy83/go-md2man/v2/md2man: cannot find package "." in:
       /Users/amake/.glide/cache/src/https-github.com-cpuguy83-go-md2man/v2/md2man
   [ERROR]	Failed to retrieve a list of dependencies: Error resolving imports
   

3. The only reasonable solution seems to be moving this project to go modules as well. In that case, the import for github.com/urfave/cli needs to be corrected to "github.com/urfave/cli/v2".

This PR moves to the go module system and fixes the import path. I did it by

1. Running go mod init, which knows how to migrate from glide
2. Fixing the import path
3. Running go mod tidy

[iwittkau]

This would also help fixing the homebrew formula.

[beauhoyt]

Hi @technosophos can you let us know what you think about this pull request. It would greatly help resolve issues we're having merging golang-1.13.5 in homebrew (Homebrew/homebrew-core#47510). Thanks~ ^_^

[technosophos]

Thanks for fixing this. I'm not a fan of Go modules, nor of the way the Go team managed this. But they are clearly unavoidable at this point. So I will move forward with them.

[beauhoyt]

@technosophos Thanks man :D