Add support for `--changed`

This PR does a few things:

commands can be run on arbitrary files instead of just .
a shortcut for running only on staged files is added with the flag --changed
- if this flag is specified and no files are staged, linting is skipped
the tool no longer exits with status code 1 when run with --fix
--fix and --changed operate independently, and can be specified together or separately
--changed works alongside specifying files, so lintlizard --changed my_file.py would run on both staged files and my_file.py

closeio / lintlizard