[gnea/grbl-Mega PR#108] DigiPots support

This pull request add supports for digital potentiometers (DigiPots) that are included on some control boards. I’ve tested this code on an Ultimachine RAMBo 1.4 board which has an AD5206 DigiPot chip controlled over SPI. This allows for software control of stepper current. By commenting out the #define HAS_DIGIPOTS, the change should be fully removed from compilation.

As a part of this code, a new per-axis setting is included to change the DigiPot value was added as $14n where n is one of the axes indexed from 0-2 for x, y, and z. @chamnit suggested in https://github.com/gnea/grbl/issues/696 that I should open a PR with DigiPots support directly onto gnea/grbl-Mega instead of gnea/grbl. We’ll probably want to consider a way that we can reserve the $14n setting so that it doesn’t become a different setting on gnea/grbl than it is on gnea/grbl-Mega.