Update the package_*_index.json process to support multiple versions

Originating in discussion in #64:

> Currently, the BM file will have only the last release which I think will work but create a "weird" user experience with the board manager utility in the IDE:

I'll reorganize it to support multiple versions, I agree that is desired.

Looking here, need to have the automated update process insert multiple version details into the primary payload rather than regenerating the entire file each time: https://adafruit.github.io/arduino-board-index/package_adafruit_index.json