add MRU_Menu_Path for defining a custom menu location

Mostly this just required turning some more things into `exe` calls.  I don't know if this was quite the right approach, but it seems to work.

Added some documentation to the top of the file and handled the default.  Anything I should do differently here?

(Great little plugin, by the way.  I just wanted to be able to put it at the top-level of menus so I'd remember to use it.)