Release plugin support

[timothyb89]

It would be nice to have (in addition to mutation/update plugin) support for release plugin. These would perform whatever tasks are needed to release a new version. Some examples:

• PR to helm git source to bump chart version, user can specify a new version number or 'major', 'minor', or 'patch' to increment the respective field by +1 set others to 0
  • CI in monasca-helm will pick up the change and automatically release once the PR is approved
• PR to docker git source to trigger rebuild
  • for images using dbuild and the master-datestamp versioning policy, just put up an empty commit with !push <module> in the commit message
  • for for semver, follow helm semantics
  • once approved, images are rebuild and pushed to docker hub, pr-bot already notifies about the change