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.

main.c

@@ -47,7 +47,7 @@ int main(void)
 {
   // Initialize system upon power-up.
   serial_init();   // Setup serial baud rate and interrupts
-  settings_init(); // Load grbl settings from EEPROM
+  settings_init(); // Load Grbl settings from EEPROM
   stepper_init();  // Configure stepper pins and interrupt timers
   system_init();   // Configure pinout pins and pin-change interrupt