[wip] Tab completion for variables only

This seems to work, but comments are certainly welcome. I am trying to write some tests, but given the nature of the feature (queuing off [TAB] and whatever is on the current buff) it's proving difficult. Do you have suggestions? 

This also seems like it could be extended a bit to start looking at class constants, application defined functions and so on. But I couldn't, just yet, figure out just how to invoke those methods and get back arrays rather than pretty print.

This also has an annoying side effect of appending a space at the end of the autocomplete.