Modify CLI to support input paths as positional arguments

[evhub]

Quoting @asottile in #20:

Then I'd suggest changing the CLI to instead be:

undebt -p pattern [-p pattern ...] FILENAME [FILENAME ...]

This is pretty standard for most linting / rewriting tools and certainly helps with xargs as above (and would make undebt compatible to use as a pre-commit hook)

[evhub]

Also quoting my comment from the previous thread:

I think I do actually like that better, although it would break backwards compatibility, so we'd want to bump the version to 0.3.0.