Re-add "twitcher" mode

[iingram]

The current tracker tries to avoid extraneous motion on target (the circling that used to happen) but perhaps at the expense of quick tracking. You might want to add a mode like before and ideally an easy way of controlling which is being used.

[iingram]

The latter bit: being able to easily control which sort of MotorController and hence the nature of the tracking was addressed in commit 42cdd79.