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`.

I collected a list of emoji from
https://github.com/ikatyang/emoji-cheat-sheet and trimmed it down. Now,
the pull request approval message will chose one of these emoji at
random to include.

This puts the cursor in the correct position to quickly add a top-level
review comment.

Add commands to quickly approve or reject a pull request.