Sent email to author participants of a Pull Request

It is possible to send email to DevelopersRecipientProvider, etc.

It should also be possible to send email to GitHub Pull Request participants.

Use case: developer submits Pull Request. Later he merges master to feature branch and waits for tests to be passed to merge Pull Request. It should be possible to notify Pull Request participants only when testing is finished. (DevelopersRecipientProvider would notify all the authors of commits merged from master)