Add a PWMInputDevice class

[dancaugherty]

Add PWMInputDevice class to the class hierarchy

Some devices like the Parallax Feedback Servo have a PWM input for detecting state changes. In the case of this device, it has a line from an internal potentiometer to read the current angle of the servo.

One possible example of a class using this device:

from gpiozero import ParallaxFeedbackServo

pfs = ParallaxFeedbackServo(CONTROL_PIN, FEEDBACK_PIN)
pfs.set_starting_angle(0)
pfs.set_speed(1).until(degrees=-45)

[dancaugherty]

For this particular device, here is an example making direct pigpio calls:
https://github.com/choeffer/360pibot/blob/master/lib_para_360_servo.py