Enable continue/back X steps.
General development. use strict.
Remove redundant getCursor from object definition.
Remove HALT movement.

It is just pointless to define HALT as a movement instruction. Halting
the turingmachine is only defined as reaching a final state (accepting
language) or running into an undefined instruction (rejecting language).
So the declaration of final states suffices for all usecases and HALT
can be left out.
Make all variables local. Complies with strict mode.