add the pr list command

This adds the ability to list the open pull requests on a single repo.

When there are more than 20 open pull requests it grabs the newest ten and oldest ten, while lightly insulting you...

This also converts the 'repo_match()' method to a mixin method as it is used multiple places.
Merge pull request #12 from PagerDuty/pr_list

Merge pull request #12 from PagerDuty/pr_list

add the pr list command