New subcommand, gb list about gb

[fatih]

This would be also helpful for editors. I'm using extensively go list to create the list of dependencies any many other things that I'm passing to go build or go install. So having it would make the integration for editors more easily.

[freeformz]

+1 also for things like heroku-buildpack-go

[mariokostelac]

@freeformz don't worry, already planning to do that :)

[davecheney]

@mariokostelac #76 contains an initial cut of gb-list, feel free to take it and run with it.