Implement <include> and <markup> tags.

Implement <include> and <markup> tags.

* lisp/muse-html.el (muse-html-markup-regexps): Make paragraph detection
  work better when a paragraph follows some other markup, especially
  tables.

* lisp/muse-publish.el (muse-publish-markup-tags): Add <include> and
  <markup> tags.
  (muse-publish-markup): Fix edge case.
  (muse-publish-markup-header-footer-tags): New option that specifies
  which tags may be used when publishing headers and footers.
  (muse-insert-file-or-string): Use
  muse-publish-markup-header-footer-tags instead of a hard-coded value.
  (muse-publish-markup-region): Make title and style arguments optional,
  so that this is easier to use.
  (muse-publish-include-tag): Implement <include> tag.  I've tested all
  of the cases I can think of, and it works.
  (muse-publish-mark-up-tag): Implement <markup> tag.  This is handy for
  times when you want to publish Muse markup in headers and footers.
  Adding a table of contents to a header should be particularly easy now.
git-archimport-id: mwolson@gnu.org--2006/muse--main--1.0--patch-143