refactor CLI code

[nsheff]

This PR adapts looper to use the form we've been using in other projects:

• moves the CLI functions to cli_looper.py and cli_divvy.py.
• cleans up the __init__.py, which is where a lot of the CLI was happening (which confused me).

I want to move toward abstracting away the CLI as only one of the possible interfaces to looper functions, and creating a separate file to hold all the CLI code makes this more clear to me.

[nsheff]

@donaldcampbelljr I have this passing all tests now (locally), so I think this is ready to merge if you approve. Can you look it over and let me know what you think?

[nsheff]

indeed, I never tested the divvy CLI. good catch!