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..

report.h

@@ -36,6 +36,7 @@
 #define STATUS_SETTING_READ_FAIL 11
 #define STATUS_HOMING_ERROR 12
 #define STATUS_ABORT_CYCLE 13
+#define STATUS_PURGE_CYCLE 14
 
 // Define Grbl feedback message codes. Less than zero to distinguish message from error.
 #define MESSAGE_SYSTEM_ALARM -1
@@ -43,7 +44,7 @@
 #define MESSAGE_HOMING_ENABLE -3
 #define MESSAGE_SWITCH_ON -4
 #define MESSAGE_SWITCH_OFF -5
-#define MESSAGE_SWITCHES_CLEARED -6
+#define MESSAGE_PURGE_AXES_LOCK -6
 
 // Prints system status messages.
 void report_status_message(uint8_t status_code);