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`.

This variable isn't necessary. The same effect can be accomplished with
the try-catch.

Ensure that diff corrections are applied in a `vimdiff` context, where
we haven't run `:diffthis` manually.

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.

If `PrrStart` hasn't been run yet, we should error and stop execution so
we don't insert newlines and start insert mode in the current buffer.

Eventually, we should set it up so `PrrStart` is called automatically
from `PrrComment` if it hasn't been already.

We already said we wanted to write a comment with the command, so we
should start in insert mode immediately rather than it being an
additional action.

The idea is to open the Prr file at the line corresponding to the
current line in the real source file.

This makes it easy to comment on a line while looking at the source,
rather than the diff.

Guard against accidentally submitting a stale unsaved code review.

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.