Add subsystem class

[andrewda]

We should at least allow for something like:

fun defaultCommand = ArcadeDrive().start()

instead of WPILib's

override fun initDefaultCommand() {
    ArcadeDrive().start()
}

[SUPERCILEX]

@andrewda That's actually my mistake. WPILib already has the setDefaultCommand(...) method which is what we should be using anyway.

[andrewda]

How about automatically extending RobotLifecycle from the subsystem class so we don't have to do it with every new subsystem?

[SUPERCILEX]

Oh, good idea!