Introduce Muse list edit minor mode

Introduce Muse list edit minor mode

* lisp/muse-mode.el (muse-mode): Don't duplicate the paragraph-start
  regexp unnecessarily.
  (muse-list-edit-minor-mode-map): New variable containing the keymap for
  Muse list edit minor mode.
  (muse-l-e-m-m-list-item-regexp): New variable that matches list items
  for Muse list edit minor mode.
  (muse-l-e-m-m-insert-list-item, muse-l-e-m-m-i-list-item-indentation)
  (muse-l-e-m-m-d-list-item-indentation): Variants of existing list item
  functions that change the list item regexp before calling the existing
  function.
  (muse-l-e-m-m-data): New variable that keeps track of the fill data
  that we overwrite when activating Muse list edit minor mode, so that we
  can restore it if the minor mode is toggled off.
  (muse-list-edit-minor-mode): New minor mode that is useful for editing
  lists in other major modes.
  (turn-on-muse-list-edit-minor-mode)
  (turn-off-muse-list-edit-minor-mode): New interactive functions that
  turn Muse list edit minor mode on and off, respectively.
git-archimport-id: mwolson@gnu.org--2006/muse--main--1.0--patch-413