Heuristically guess username from gh auth status

[norwd]

This closes #2 by running gh auth status and parsing its output for a username.

The output of gh auth status prints a little summary of the current gh session, including the GitHub username:

I've tested this using the latest version on gh on a Linux box, so I don't know how this will behave on MacOS or Windows for example, but I've designed it to simply fall back to the old behaviour of expecting an explicit argument if it can't find anything that looks like a username, so it fails gracefully.

[rsese]

Thanks! Seems like a nice default -- I gave this a try and it works for me on macOS.