Move regexps to same group, only make 4 heading faces.

Move regexps to same group, only make 4 heading faces.

* muse-colors.el (muse-make-faces): Only create 4 header faces since we
  can only publish 3 of them specially, leaving the fourth to be in
  plain text.
  (muse-colors-markup): Only recognize 4 levels of heading.

* muse-regexps.el (muse-tag-regexp, muse-link-regexp, muse-url-regexp)
  (muse-file-regexp, muse-image-regexp)
  (muse-ignored-extensions-regexp): Moved here from muse.el.  It seems
  like a good idea to have all the regexps in the same group.
git-archimport-id: mwolson@gnu.org--2005/muse--main--1.0--patch-48