Reply to a comment at lowest legal point in emacs.
Reply to a comment at lowest legal point in emacs.

So that newer replies start lower than older replies to the same
comment.

Vim already has this behavior.

Incidentally, fix some of the font-lock rules for bodies at the ends
of files.

Closes #18.

tomheon/diffscuss,tomheon/diffscuss,hut8labs/diffscuss,hut8labs/diffscuss