Improved homing cycle. New settings: homing enable/rates, debounce and step idle lock time.

- Homing cycle will now cycle twice (spec more/less in config) to
improve repeatability and accuracy by decreasing overshoot.

- New Grbl settings added: Enable/disable homing cycles, homing seek
and feed rates, switch debounce delay, and stepper idle lock time.

- Please note that these settings may change upon the next push, since
there will be more added soon. Grbl *should* not re-write your old
settings, just re-write the new ones. So, make sure you keep these
written down somewhere in case they get lost from a code bug.

- Refactored settings migration to be a little smaller and managable
going forward.

stepper.c

@@ -112,9 +112,7 @@ void st_go_idle()
   TIMSK1 &= ~(1<<OCIE1A); 
   // Force stepper dwell to lock axes for a defined amount of time to ensure the axes come to a complete
   // stop and not drift from residual inertial forces at the end of the last movement.
-  #ifdef STEPPER_IDLE_LOCK_TIME
-    _delay_ms(STEPPER_IDLE_LOCK_TIME);   
-  #endif
+  delay_ms(settings.stepper_idle_lock_time);   
   // Disable steppers by setting stepper disable
   STEPPERS_DISABLE_PORT |= (1<<STEPPERS_DISABLE_BIT);
 }