FWRETRACT in mm/s

Firmware retraction now stores the retract and recover speeds in mm/s
instead of mm/min. This makes it match the units of the maximum
feedrate, and fixes problems with modifying the value via LCD control
panel. From gcode, the values are still taken in mm/min to match the
units of G1 and similar, and they are converted to mm/s before they are
stored.

I also lowered the default retract feedrate to make it less likely to
cause problems for geared extruders when the user hasn’t bothered to
set a reasonable maximum feedrate, though users should be setting both
of these values to suit their hardware.
fix bug in M200 with multiple extruders
Merge pull request #837 from whosawhatsis/fwretract

FWRETRACT in mm/s
Update Readme descriptions for M200, M207 and M208
Merge pull request #871 from whosawhatsis/fwretract

Update Readme descriptions for M200, M207 and M208
Adjustable Z probe offset, via custom M-code
Allowable range now includes default value
use existing strings
Merge pull request #804 from MissionSt/adjustable-z-probe-offset

Adjustable Z probe offset via custom M-code