Basic support for filling of definitions

This is related to the definition list extension as defined by PHP
Markdown Extra, Pandoc, and so on.  This patch prevents definitions from
collapsing with the previous line (the defined term) when filling.

This closes the following issue:
#20

Thanks to Marijn Haverbeke (@marijnh) for the patch.

markdown-mode.el

@@ -5090,6 +5090,7 @@ before regenerating font-lock rules for extensions."
                     "[ \t]*[*+-][ \t]+" ; unordered list item
                     "[ \t]*\\(?:[0-9]+\\|#\\)\\.[ \t]+" ; ordered list item
                     "[ \t]*\\[\\S-*\\]:[ \t]+" ; link ref def
+                    "[ \t]*:[ \t]+" ; definition
                     )
                   "\\|"))
   (set