Add a way to auto-configure git-risk for a repository

We should have some method of automatically configuring `git-risk` for a given repository (i.e. adding a section to `.git/config`). It would, of course, use the default ticket regular expression, but this would probably be ok to get started for users.

Start by reading the git-risk entry point and checking how it currently accepts repository configuration. Implement a way to add the default ticket regular expression to the repository's .git/config, then verify that a configured repository can run git-risk without additional setup.

Written by the indexing model from the issue text.