Turbo/Turtle Triggers

[Toasterman9000]

It is customary due to various reasons to add turbo/turtle controls. To elaborate, you want to go twice as fast when you are holding down the right trigger and you want to go half as fast when you are holding down the left trigger.
This should be a pretty simple addition through some if-then statements. Do it however you feel is best.

[prosa100]

nice job on tank drive. please implement for mechanum.