Support GitHub Deployment Statuses as a Notification target

Not quite sure what the syntax would look like, but it would be nice to put something like

  - github:
      environment: 'production'

Though, now that I look at the api, that would have to hit at least two github endpoints and maybe makes more sense as a plugin rather than a notify step.

Maybe the notify statement could naively do a Create Deployment and a Create Deployment Status with a state of success?