Fix gometalinter flakes by setting CGO_ENABLED=0

[dnephin]

I looked into the lint CI failures in #17 by re-running the job with ssh enabled. In the shell I ran

gometalinter --debug --vendor ./...

In the debug output I noticed

/go/bin/structcheck returned exit status 1: exec: "gcc": executable file not found in $PATH

Often (maybe always) when you get linter errors reported for /usr/local/go this indicates that linting is failing because go build failed.

The gometalinter docker image uses alpine and doesn't include gcc and all the c header files, so running with CGO_ENABLED=0 fixes the build by using the pure-go compiler.

Also added a config file to explicitly define and configure linters. I added a few additional linters that aren't in the default set but which I find helpful (gosimple, misspell, goimports, nakedret, unparam).

I also removed dupl from the list, which is enabled by default. In my experience the warnings this reports are often not very valuable, but I would be happy to re-enabled it if you would like.

[dnephin]

This is just for convenience. It adds the -w flags for these two commands so that if you run it locally, and there are errors, the fixes for the errors will be written.

[hannahhenderson]

lgtm, though my Go-knowledge is limited

[marcomorain]

Awesome, thanks @dnephin