Support PlatformIO

[Kaspi314]

For the time being I will simply copy this into the library folder for projects but please consider supporting PlatformIO at some point down the road.

I spent the morning trying to figure out how to do that and only came up with a partial idea:
https://docs.platformio.org/en/latest/boards/atmelavr/attiny85.html#board-atmelavr-attiny85

I'll just leave this here in case you ever get around to it or I figure something better out.

https://github.com/platformio/platform-atmelavr/blob/develop/boards/attiny85.json

This seems that it would be added as a "framework" instead of "arduino" but I'm not sure how to specify with picoboot ("protocol": "usbtiny") or usbasp. (maybe an array?)