Rebuild .gh status feature

[dgw]

In #138 I discovered a broken feature, .gh status, and restored it to minimal functionality. It appears to have been more informative in the past, but GitHub's status page must have changed providers or their provider evolved the available API endpoints and I couldn't get a good sense of how it used to work, quickly.

There are a couple of useful looking endpoints in the current status API, though:

1. Unresolved incidents
2. Active scheduled maintenances

Info from those could be useful if the status indicator is not "none" (no active incidents).

Separately, it might be worth moving this feature to its own .gh-status command.