Tweaks and minor bug fixes. Added purge buffer command.

- Added a purge buffer (and lock) command. This is an advanced option
to clear any queued blocks in the buffer in the event of system
position being lost or homed. These queued blocks will likely not move
correctly if not purged. In typical use, the purging command releases
the homing axes lock in case a user need to move the axes off their
hard limit switches, but position is not guaranteed. Homing is advised
immediately after.

- Created a system-wide sync current position function. Cleans up some
of the repetitive tasks in various places in the code that do the same
thing.

- Removed the clear all switches command '$S'. Not really needed and
helped clean up a sync call.

- Other minor tweaks. Readme updated slightly..

planner.h

@@ -70,9 +70,6 @@ block_t *plan_get_current_block();
 // Reset the planner position vector (in steps)
 void plan_set_current_position(int32_t x, int32_t y, int32_t z);
 
-// Clear the planner position vector
-void plan_clear_position();
-
 // Reinitialize plan with a partially completed block
 void plan_cycle_reinitialize(int32_t step_events_remaining);