Add notifier structure to support multiple kinds of notifiers.

Before, drill sergeant was hardcoded to only support email notifications of stale pull requests.

This new structure now can support any number of different style notifiers (and perhaps in the future support user supplied notifiers). As part of this new structure, a new notifier has been created for Github which notifies by attaching a "Stale" label to the PR that is considered stale.

Now that there are more than one provider for notifications, email is now an optional argument (provided that the `label` argument is supplied at run time).