add file-based config #11

[jaymcgrath]

Hi @JoranHonig - I'm opening this PR for discussion of my implementation of issue #11.

I agree that adding an unmaintained dependency would be bad. I've taken your suggestion and worked up this implementation of basic file-based config, using pyYAML's safe_load to load the yaml into a dict.

I'm using dict.get to avoid KeyErrors and return defaults that match the ones click returns.

Before I polish this up and add test coverage and catch exceptions around validating the yml file, I wanted to run this by you to see what you thought.

Thanks!

[jaymcgrath]

Next steps:
Examine PyYAML to determine which exceptions it raises - wrap this in a try/catch
Format and include an example yml config file
Update docs
Add tests

Possible enhancements:
Currently, we assume that the file is in the current working directory. Maybe we should try to load it as an absolute path, then if that fails, prepend the cwd and try again?

[JoranHonig]

@jaymcgrath Thanks for the contribution! 🎉

I'm travelling for the first few days of this week so I might not get to review your comments & pr until thursday