Add gofmt_command option

It helps in case you want to use [goimports](https://godoc.org/code.google.com/p/go.tools/cmd/goimports) for example.

I consider it a draft since there are no docs and maybe I'm missing something else too. 

- Should I add something else apart from docs?
- Is there are a reason why the other options we have don't follow the usual convention of `g:plugin_foo` (I might be wrong about considering it a convention my experience with vim plugins is pretty limited)