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/gcode.c

@@ -983,10 +983,10 @@ uint8_t gc_execute_line(char *line)
   // [19. Go to predefined position, Set G10, or Set axis offsets ]:
   switch(gc_block.non_modal_command) {
     case NON_MODAL_SET_COORDINATE_DATA:
-      settings_write_coord_data(coord_select,gc_block.values.xyz);
+      settings_write_coord_data(coord_select,gc_block.values.ijk);
       // Update system coordinate system if currently active.
       if (gc_state.modal.coord_select == coord_select) {
-        memcpy(gc_state.coord_system,gc_block.values.xyz,N_AXIS*sizeof(float));
+        memcpy(gc_state.coord_system,gc_block.values.ijk,N_AXIS*sizeof(float));
         system_flag_wco_change();
       }
       break;