Add servo examples not dependent on PCA9685

[dhalbert]

A servo can be driven directly from a PWM pin. Add examples for regular servos and continuous servos, something like https://learn.adafruit.com/adafruit-circuit-playground-express/circuitpython-servo.

[ladyada]

@kattni wanna write this?

[ladyada]

added one here
#27

[caternuson]

I think #27 took care of this? If not reopen.