Jit-lock support for draft-mode highlighting

Use jit-lock-mode in draft mode, which gives instantaneous highlighting.  This is a much more pleasant experience than the existing alternatives.

The first commit changes the algorithm used for computing citation prefix faces to one that is stateless.  Thanks to ikazuhiro's suggestion, there is no reduction of functionality, and the added benefit is that the choice of colours is now deterministic (the chosen colours won't change as you edit the message).

As far as I know, there are only two cases when the user will need to hit C-l:  modifying a multi-line
citation line so that it is no longer recognised as one, and modifying a signature separator so that it stops being one.  In both cases, the no-longer-matching area will remain highlighted.  I think that's a fairly minor issue, and as fixing it would require some hackery with text properties (in order to widen the highlighted area appropriately), I'm not going to bother.

-- Juliusz