[imp] Turn quote-level highlighting and signature dimming into user prefs

[Chuck Hagenbuch]

Quoting "Marcus I. Ryan" <marcus@riboflavin.net>:

> Here's a patch...see subject.  Basically it adds the preferences to
> prefs.php.dist and then encloses the appropriate code in text.php in if
> statements...

Committed, thanks!

-chuck

--
Charles Hagenbuch, <chuck@horde.org>
"A dream which helps you to live your reality with dignity
 and justice is a good dream." - Tariq Ramadan