[WIP] Merge all Pull Requests in a repo

Closes #75

I wanted to see if this was the right direction for what you had in mind. If so I can finish it up by writing tests.

Let me know!
Right, but where [the `repository` option](https://github.com/education/teachers_pet/pull/77/files#diff-1a4cd0ab0892ad252d9821f61d01d781R3) is specified, the "banner" (that is, the helper text) says the flag should be given as `--repository=OWNER/REPO`.  This is inconsistent between commands, where some ask for the `repository` to be prefixed with `OWNER/` and others don't, so that's something that needs to be resolved in general :confused: