Support Labels RxChar for local display

[hathach]

First of all, thank you very much for this great and fun project.

For context, I have ported the motion kit sketch to work with Adafruit nRF52 core to run on CLUE and Feather Sense boards. And it works great with lots of fun. CLUE has an builtin TFT, which can be very useful to display label on nrf52. However, currently the label is not available to devices (maybe I missed it, but I couldn't find them anywhere) and I need to "hardcorded" the label to use with "Air Snare" and "Finger User Interface" https://github.com/adafruit/tf4micro-motion-kit/blob/adafruit/arduino/adafruit/tf4micro-motion-kit/tf4micro-motion-kit.ino#L143

It would be great to have an LabelRxChar so that server could pass it to nrf52 device. Something simple as comma separated string would be more than enough. Since the string can get to very long (with lost of labels) while characteristic length can only be up to 512. Therefore the detail of implementation is totally up to you.

CLUE-motion-kit.mov

@ladyada

PS: If you are ok with the comma separated char, I am happy to make an PR for it.