Adding support for Feather ThinkInk

[BlitzCityDIY]

Adding support for the Feather ThinkInk. Tested with u2if firmware

[makermelissa]

I added @caternuson as a reviewer since he originally added all the U2IF boards.

[caternuson]

Looks like this is the associated Platform Detect PR, adding for general ref: https://github.com/adafruit/Adafruit_Python_PlatformDetect/pull/286

@BlitzCityDIY Did not test (do not have hardware), but looks generally OK. One missing pin define for A3. What actual u2if firmware was used for testing?

[BlitzCityDIY]

thanks @caternuson - just added A3. i have a fork here for u2if (haven't PR'd yet): https://github.com/BlitzCityDIY/u2if

[caternuson]

oh cool - you built one locally. awesome. lgtm.