Add Adafruit_Feather footprints

One version with mounting holes, one version without.

See https://learn.adafruit.com/adafruit-feather-m0-basic-proto for the basic info on an example Feather board, and https://learn.adafruit.com/adafruit-feather/feather-specification for details on the layout of Feather boards in general. The corresponding symbol is added in another PR.

Pin numbering is fairly arbitrary (counterclockwise with the USB connector down, like the similarly sized Arduino Nano), since the part does not have its own pin numbering (in the Adafruit schematic, both pin headers are numbered separately).

Since there is no well-defined pin 1 on the part itself, I have left out pin 1 markings from Fab and Silk layers. I did put a square pad on the copper layer, though. The assymmetric nature of the pin headers should make it clear how to rotate the board when inserting it, even without an obvious pin marking.

Checking gives one warning: The version with mounting holes has the same 3D model name as the one without, since I'd expect that the mounting holes would not be represented in the 3D model when it is created (just the pin headers).