Fix issue with table of contents and derived html styles.
* lisp/muse-html.el (muse-html-finalize-buffer): Since the html style
does not derive from any other class, return `t' here. This, along
with the corresponding change to muse-publish.el, fixes a problem with
the table of contents getting inserted multiple times for custom
html-based styles.
* lisp/muse-publish.el (muse-style-run-hooks): Make sure that we do not
run the same function twice.
git-archimport-id: mwolson@gnu.org--2006/muse--main--1.0--patch-181