Add Rotary Encoder & GPIO Interrupt support

To use in CircuitPython, simply install the Adafruit CircuitPython bundle.

To use in CPython, pip3 install adafruit-circuitpython-seesaw.

Read the docs for info on how to use it.

Changes since last release:

• Add method for reading GPIO interrupt flag (thanks @rianadon)
• Add rotary encoder support (thanks @jfurcean)
• Increase duplicate code check threshold (thanks @dherrada)