New jog cancel real-time command. Parser typo fix from last push.

- Added a new jog cancel real-time command. Rather than depending on a
feed hold to cancel a jogging motion, this realtime command can be used
instead. The main advantage is if a feed hold is used, you can
accidentally hold the machine right when Grbl returns to IDLE after
completing a jog. And the GUI doesn’t have to worry about tracking this
either.

- Fixed a typo in the g-code parser edits from the last push. Was
causing the G10 set coordinate system command to not work correctly.

- Updated the documentation with the jog cancel command.

grbl/serial.c

@@ -157,6 +157,11 @@ ISR(SERIAL_RX)
       if (data > 0x7F) { // Real-time control characters are extended ACSII only.
         switch(data) {
           case CMD_SAFETY_DOOR:   system_set_exec_state_flag(EXEC_SAFETY_DOOR); break; // Set as true
+          case CMD_JOG_CANCEL:   
+            if (sys.state & STATE_JOG) { // Block all other states from invoking motion cancel.
+              system_set_exec_state_flag(EXEC_MOTION_CANCEL); 
+            }
+            break; 
           #ifdef DEBUG
             case CMD_DEBUG_REPORT: {uint8_t sreg = SREG; cli(); bit_true(sys_rt_exec_debug,EXEC_DEBUG_REPORT); SREG = sreg;} break;
           #endif