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, but I think it's definitely a step in the right direction, and maybe even good enough to merge.  Feedback welcome.

TODO list:

* each comment on an issue or PR results in the same line in the report
* if you create a PR, it's probably not worth mentioning that you commented on it
* work on related repos (origin and fork) need to be grouped together
* all work on a pull request needs to be grouped together
* rebasing or fast-forwarding a personal fork branch and pushing it generates uninteresting events
* repeated `git push -f` cause a lot of duplicate info
* ensure items within a single list are always chronologically sorted
* add repo inclusion / exclusion lists