ghprb - Allow downstream jobs to add GitHub status messages

Currently the GHPRB allows you to add a custom GitHub status and context to the job which performs the pull request. We need a way to allow downstream jobs to also add a context and status to the same pull request.

To accomplish this a checkbox is added to the "Build Environment" area of a Jenkins Job that says "Set build status for Github Pull Request". It should grab the pull request from the upstream job and allow the downstream job to attach a custom status message and context.