Add detection for XHTML

[onecrayon]

Currently, whatever is the default in the preferences is what is always used. I need to figure out a way to detect XHTML vs. HTML as often as possible, and only default to what's in preferences if nothing is detected.

[sergeche]

I'm using DOCTYPE detection for Aptana: if there's XHTML Doctype or declaration, I'm using XHTML output profile

[onecrayon]

Yeah, detecting doctype will definitely be part of the puzzle. I'm thinking I might try to see if the file contains self-closing single tags, as well.