Accept a range of commits as an argument to `gitcoach`

The primary use case that I can think of for this is code review on pull requests:
- Pull requests consist of multiple commits. 
- Each commit touches multiple files.

If I specify the range of commits included in a pull request, `gitcoach` should be able to suggest files that ought to have been modified as well. Thus `gitcoach` becomes useful for code review _and_ pre-commit sanity checking.
