Move to beginning of line

Hitting 0 takes you to the beginning of the line. When 0 is hit, we check to see if we're inside of a repeat (checking with `topOperator`). If so, we pass control over to `repeatPrefix`. Otherwise, we move to the beginning of the line. I originally attempted to use `e.abortKeyBinding()`, but `repeatPrefix` didn't ever fire. Not sure if that's a bug or my misunderstanding. All tests pass.