Add threshold.

[caternuson]

Add a property to query and set threshold that is compliant with touchio API:
https://circuitpython.readthedocs.io/en/3.x/shared-bindings/touchio/TouchIn.html#touchio.TouchIn.threshold

Since threshold can be set per channel, suggest having both a global and a per channel way of doing this, similar to how recalibrate is now. Ex:

cap.threshold = some_value     # set threshold for all inputs
cap[1].threshold = some_value  # set threshold for input 1

[caternuson]

Done with #7