Make resolving of inline image paths more flexible.

Make resolving of inline image paths more flexible.

* lisp/muse-colors.el (muse-colors-inline-image-method): Rename from
  `muse-inline-relative-to'.  Add muse-colors-use-publishing-directory
  option.  Make the fallback customization type a function rather than a
  symbol.
  (muse-colors-use-publishing-directory): New function that takes the
  best guess at where the current Muse file will be published and makes
  the given link relative to that.
  (muse-colors-insert-image): If muse-colors-inline-image-method is
  'default-directory, use the default directory.  If it is a function,
  call that function with LINK as an argument.  Otherwise, give up.
git-archimport-id: mwolson@gnu.org--2006/muse--main--1.0--patch-50