Improve stencil generation to clean all surface of pads

[filippsudanov]

There's an approach to make PCB soldermask by UV curable glue and clean this glue away by CNC maching with floating bit - e.g. see this video: https://youtu.be/7EctuSTL7-w?t=3m38s

For this we need a toolpath that will mill entire surface of pads. Current stencil function only mills around edges of pads.

Such toolpath generation is currently implemented in http://chilipeppr.com/grbl, but it would be cool if someone could include it into pcb-gcode.