Support setting PR status

I don't doubt that this is already on the radar for the vsts-cli team, but supporting setting PR statuses will be a very valuable addition to the supported commands. For our environment, it's actually the only REST API call we're really using right now so that we can block PR completion until an external build/test process has finished.

For general reference, the REST API docs for PR statuses is here:
https://www.visualstudio.com/en-us/docs/integrate/api/git/pull-requests/pullrequeststatuses

Thanks!