Added enhanced list item indentation.

Added enhanced list item indentation.

* lisp/muse-mode.el (muse-mode-map): Added key mappings to increase or
  decrease the current item indentation.

* muse-mode.el (muse-get-paragraph-start): Helper function to determine
  the start of the current paragraph.

* muse-mode.el (muse-insert-list-item): Cleaned up list item insertion
  code and made it handle definitions.

* muse-mode.el (muse-increase-list-item-indentation): New interactive
  function to increase list item indentation.

* muse-mode.el (muse-decrease-list-item-indentation): New interactive
  function to decrease list item indentation.

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Files to commit:
   lisp/muse-mode.el

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git-archimport-id: mwolson@gnu.org--2006-muse-el/muse-el--devel--0--patch-14