Add general searching and backlink searching.

Add general searching and backlink searching.

* lisp/muse-mode.el (muse-grep-command): Add custom to set up the command
  used for grepping.

* muse-mode.el (muse-mode-map): Changed muse-browse-result to C-c C-v and
  made muse-find-backlinks C-c C-b

* muse-mode.el (muse-grep): New function to envoke a search with the compiler.

* muse-mode.el (muse-search-with-command): New function to search for
  text, optionally editing the command.

* muse-mode.el (muse-search): New function to search for text.

* muse-mode.el (muse-find-backlinks): New function to search for
  backlinks to the current page.

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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-25