colors: Colorize comments and forbid tags in #title
* lisp/muse-colors.el (muse-colors-emphasized, muse-colors-underlined)
(muse-colors-verbatim, muse-colors-custom-tags)
(muse-colors-explicit-link, muse-colors-implicit-link): Don't trod on
comments or the #title directive.
(muse-colors-markup): Add comment rule.
(muse-colors-implicit-link): Only remove flyspell overlay after we
decide whether this is an implicit link, rather than before.
(muse-colors-title): Add the muse-directive-or-comment property.
(muse-colors-comment): New function that colorizes comments using
font-lock-comment-face.