Make implicit URL descriptions publish better.

Make implicit URL descriptions publish better.

* lisp/muse-publish.el (muse-publish-this-file): Display message
  if the buffer is not associated with any file, so that we avoid
  errors later on.
  (muse-publish-url-desc): New function taken from muse-publish-url
  that causes a URL description to be transformed.
  (muse-publish-url): Call muse-publish-url-desc on either the
  description or the original URL if it will be used as a
  description.  Accept the original URL as an argument, in case it
  was transformed earlier.
  (muse-publish-insert-url): Pass original URL as an argument.
  (muse-publish-markup-link): Make this somewhat easier to follow.
  Pass original URL as argument.
git-archimport-id: mwolson@gnu.org--2006/muse--main--1.0--patch-185