Add configurable markdown path todo item.

doc/todo/configurable_markdown_path.mdwn

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+[[!template id=gitbranch branch=wtk/mdwn author="[[wtk]]"]]
+
+summary
+=======
+
+Make it easy to configure the Markdown implementation used by the
+[[plugins/mdwn]] plugin.  With this patch, you can set the path to an
+external Markdown executable in your ikiwiki config file.  If you do
+not set a path, the plugin will use the usual config options to
+determine which Perl module to use.