Add support for Github commit status

*For review and testing*

Github supports a [commit status feature](https://github.com/blog/1227-commit-status-api).

This adds a new `--set-commit-status` flag to the `merge` command, which will create a success or failure status on the latest commit of all PRs in the merge, depending if all PRs could be merged or not.

It also adds a `set-commit-status` command that takes options similar to the `merge` command in addition to `status`, `message`, and `url` options.  It uses the same logic as `merge` to find which PRs apply and then sets the given commit status on those PRs.  `--info` is supported to see which PRs apply without setting the status.