Add Support for Grbl Boards

[RylanOC]

The XY Table I'm using is Grbl based - rather than replacing the board with a Marlin based one, I updated the move and sweep functions to support Grbl queries and responses. I do not have a Marlin based board to test on to ensure that the library still works with the default Marlin based board, however the following code snippet should perform a sweep on a Grbl based board:

from chipshover import ChipShover, FirmwareType
shv = ChipShover('/dev/tty.usbserial-14330', firmware=FirmwareType.Grbl)

for x,y in shv.sweep_x_y(10, 20, 10, 20, step=0.05, z_plunge=1.5):
    print("At %f, %f"%(x,y))

Other functionality (ie. fan control, stopping/killing a job, homing, etc.) are still unsupported on Grbl based boards.