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

[tarmstrong]

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.