shell-function-invoker -> command-function-invoker

[scothis]

Next steps:

• rename GitHub repo
• update Travis, if needed
• update riff-cli
• update FATS

Issue #5

[scothis]

CI is expected to fail since the new package isn't on the go path by default, the repo will need to be renamed first.

[ericbottard]

I don't actually understand the travis failure, as it is building that very package.

Let me merge this anyway, and work all the way up until everything is fixed