Sometimes I don't have any line comments to make and can go ahead and
accept or reject a change immediately.

In those cases, I want `PrrApprove` and `PrrReject` to be available in
any file type buffer, not just in *.prr buffers.

Additionally, those two commands should automatically do `PrrStart` if
it hasn't been executed manually before, like we already do with
`PrrComment`.

If the Prr session hasn't been started yet, do so automatically when
running `PrrComment`. This lets me comment directly on a line without
having to worry about starting the Prr review beforehand.

Now that I see the code and my earlier TODO, I think I could probably
have made it work without the `s:active` flag. Will have to try removing
it and testing whether that works.

Guard against accidentally submitting a stale unsaved code review.

Make a plugin to facilitate Prr code reviews from within Vim.