Minor bug fix and CPU pin map update.

- Sometime I HATE github. This push is just here to be able to describe
the last two pushes today that had a merging conflict and lost all of
the commit comments before.

- Setting $10 would cause $11 to be the same value. Missed a break
statement in the switch-case. Oops! Now fixed.

- CPU pin map for the future versions of Grbl introduced, but not yet
finalized. Still working on it and it’s subject to change.

- Added a new high-speed machining test g-code routine written by Todd
Fleming. Mills a pocket at 15,000mm/min. Do not use when connected to
your machine! You’ve been warned!