Give ourselves a little more flexibility about math output under XHTML.

Give ourselves a little more flexibility about math output under XHTML.
I think it will be worth implementing a version of pure HTML output, for
some uses, a la eLyXer.

Note that at present none of this does anything, and there is no UI to
set it. I just want to make sure it's in the file format, in case I do
not get to this before 2.0.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@33794 a592a061-630c-0410-9148-cb99ea01b6c8