bind-keys macro changed to allow prefix map to have a menu string.

I have modified the bind-keys macro so that a menu string can be added to a prefix map.  Thus:

	      (bind-keys :prefix "s-o"
			 :prefix-map s-o-outline-map
			 :prefix-docstring "map for outline --->"
			 :menu-name "Outline-"
			 ("q" . hide-sublevels)    ; Hide all but the top-level headings
			 ("t" . hide-body)         ; Hide all but headings 
              etc.....

adds the key binding names to the Outline menu.