Feature: pull request reviews and comments

Hi there! I saw your [presentation at Boston Python](https://www.meetup.com/bostonpython/events/271887195). I'm already a light user of Datasette (thank you!), but wasn't aware of this project.

I've been working on a "pull request dashboard" to get a comprehensive view of the state of open PR's, esp. related to reviews (i.e., pending, approved, changes requested). Currently it's a CLI command, but I thought a Datasette UI might be fun.

I see that PR's are available from the `issues` command, but I don't see reviews anywhere. From the [API docs](https://docs.github.com/en/rest/reference/pulls#reviews), it looks like there are separate endpoints for those (as well as pull requests in general). What do you think about adding that? Would you accept a PR? Any sense of the level of effort?