Attempt to make github plugin more useful.

Rather than merely showing counts ("I did 731 pull requests this
week, look how amazing I am!"), show what was actually worked on.

This is probably too verbose, and needs some extra de-duplication
intelligence (e.g. each comment on an issue or PR results in the
same line in the report, and if you create a PR, it's probably
not worth mentioning that you commented on it), but I think it's
definitely a step in the right direction, and maybe even good
enough to merge.  Feedback welcome.