Make it work with htmlize.

[Fanael]

Didn't test it very thoroughly, but I haven't noticed any regressions yet.

Fixes #16.

[purcell]

Good idea. Those defconsts look a bit odd to me. Not sure I understand why they're necessary.

[Fanael]

@purcell: Right, I guess it's cargo-cult programming on my part. Emacs's own font-lock.el does use similar declarations, but there's a remark right above them:

;; Note that in new code, in the vast majority of cases there is no
;; need to create variables that specify face names.  Simply using
;; faces directly is enough.  Font-lock is not a template to be
;; followed in this area.

[purcell]

That looks better! :-)