ServoKit: Add an interface to set min_pulse and max_pulse

The interface for objects created with ServoKit include settings for the i2c address, reference clock speed, and pulse frequency. What's missing in order to fully calibrate a servo is the ability to set the min_pulse and max_pulse parameters.
```
class ServoKit:
def __init__(
self,
*,
channels: int,
i2c: Optional[I2C] = None,
address: int = 0x40,
reference_clock_speed: int = 25000000,
frequency: int = 50
) -> None:
if channels not in [8, 16]:
raise ValueError("servo_channels must be 8 or 16!")
self._items = [None] * channels
self._channels = channels
if i2c is None:
i2c = board.I2C()
self._pca = PCA9685(
i2c, address=address, reference_clock_speed=reference_clock_speed
)
self._pca.frequency = frequency

self._servo = _Servo(self)
self._continuous_servo = _ContinuousServo(self)
```

It may be possible to get to the min_pulse and max_pulse parameters by referencing down into the Servo and adafruit_motor classes, but this isn't simple or obvious in the way it could be.

Start at the ServoKit.__init__ entry point and inspect how _Servo and _ContinuousServo expose their underlying servo objects. Read the referenced Servo and adafruit_motor classes to understand the existing min_pulse and max_pulse parameters. Done means ServoKit-created objects provide a clear interface for setting both calibration values.

Written by the indexing model from the issue text.