Cache PR data requests in localstorage

[rjmunro]

The interesting data for this project won't change once a PR is closed, so why not cache it indefinitely in localstorage?

This would mean that if you hit the unauthenticated rate limit, you can come back after an hour and get another 59 PRs, which would then be cached, rather than the same most recent 59 PRs again. After a few hours, you could have the complete history for a project with a lot of PRs.

[pgjones]

It is a good idea and I'd welcome a pull request, but I haven't worked on this project for a couple of years and I'm unlikely to at the moment.