Program stop support (M0,M1*,M2,M30*), proper position retainment upon reset, misc minor updates.

- Program stop support (M0,M1*,M2,M30*). *Optional stop to be done.
*Pallet shuttle not supported.

- Work position is set equal to machine position upon reset, as
according to NIST RS274-NGC guidelines. G92 is disabled.

- Renamed mc_set_current_position() to mc_set_coordinate_offset().

- Fixed bug in plan_synchronize(). Would exit right before last step is
finished and caused issues with program stops. Now fixed.

- Spindle now stops upon a run-time abort command.

- Updated readme and misc upkeeping.

gcode.h

@@ -29,4 +29,7 @@ void gc_init();
 // Execute one block of rs275/ngc/g-code
 uint8_t gc_execute_line(char *line);
 
+// Set g-code parser position. Input in steps.
+void gc_set_current_position(int32_t x, int32_t y, int32_t z); 
+
 #endif