Implement subcommand mechanism

[ikesyo]

See Carthage/Carthage#1017 for the context.

Cargo in particular has a plugin architecture, which led to the formation of a bunch of useful sub-commands.

I think carthage would greatly benefit from the addition of either support for similar plugins or the addition of similar sub-commands by default.

[mdiep]

I want there to be a better way to do this… but I guess there's probably not? /cc @Carthage/carthage

[ikesyo]

I will update this to use which -s and check the exit status.

[ikesyo]

Updated. ✨ How do you feel now?

[mdiep]

This looks and works great. 👍 Thanks!

[ikesyo]

Thanks too! 😄