Fully configurable pins for NO or NC switches.

- All pins, which include limits, control command, and probe pins, can
now all be configured to trigger as active-low or active-high and
whether the pin has its internal pull-up resistor enabled. This should
allow for just about all types of NO and NC switch configurations.

- The probe pin invert setting hasn’t been added to the Grbl settings,
like the others, and will have to wait until v1.0. But for now, it’s
available as a compile-time option in config.h.

- Fixed a variable spindle bug.

planner.c

@@ -371,7 +371,7 @@ uint8_t plan_check_full_buffer()
        colinear with the circle center. The circular segment joining the two paths represents the 
        path of centripetal acceleration. Solve for max velocity based on max acceleration about the
        radius of the circle, defined indirectly by junction deviation. This may be also viewed as 
-       path width or max_jerk in the previous grbl version. This approach does not actually deviate 
+       path width or max_jerk in the previous Grbl version. This approach does not actually deviate 
        from path, but used as a robust way to compute cornering speeds, as it takes into account the
        nonlinearities of both the junction angle and junction velocity.