Explore Go command line argument parsing libs that easily allow the definition of subcommands

[therapon]

Are there any Go libraries that you can recommend on command line argument parsing? Especially with the ability to define sub-commands (ala git's CLI)?

The test driver specification is a DocOpt string and it looks like there is a Go implementation https://github.com/docopt/docopt.go which should make the argument parsing task easy?

Originally posted by @therapon in https://github.com/amzn/ion-go/pull/47/files