Add `CommandTMRU` functionality to Lua version

[pokeylope]

The Lua implementation is missing :CommandTMRU/<Plug>(CommandTMRU), which displayed the Command-T buffer window sorted in most-recently-used order.

[wincent]

This hasn't been prioritized because I never used the MRU finder myself, but I'd be open to a pull request (and even in the absence of a pull request, I do intend to get to it eventually).