Integrated tab completion for both commands and filenames.  Example, this invokes the :tabnew command and starts filename completion::  :tab<tab><tab>
Commands like :r<tab> or :r something<tab> now invoke *filename* completion. Ditto for :tabnew.
All tests pass, but this is still experimental code.  If there are problems, set new_arg = False at the start of leoKeys.py.

Leo build: 20140818173142