G54 work coordinate support (w/ G10,G92.1). Re-factored g-code parser with error checking. Minor compiler compatibility changes.

- G54 work coordinate system support. Up to 6 work coordinate systems
(G54-G59) available as a compile-time option.

- G10 command added to set work coordinate offsets from machine
position.

- G92/G92.1 position offsets and cancellation support. Properly follows
NIST standard rules with other systems.

- G53 absolute override now works correctly with new coordinate systems.

- Revamped g-code parser with robust error checking. Providing user
feedback with bad commands. Follows NIST standards.

- Planner module slightly changed to only expected position movements
in terms of machine coordinates only. This was to simplify coordinate
system handling, which is done solely by the g-code parser.

- Upon grbl system abort, machine position and work positions are
retained, while G92 offsets are reset per NIST standards.

- Compiler compatibility update for _delay_us().

- Updated README.

limits.c

@@ -76,9 +76,9 @@ static void homing_cycle(bool x_axis, bool y_axis, bool z_axis, bool reverse_dir
     // Check if we are done
     if(!(x_axis || y_axis || z_axis)) { return; }
     STEPPING_PORT |= out_bits & STEP_MASK;
-    _delay_us(settings.pulse_microseconds);
+    delay_us(settings.pulse_microseconds);
     STEPPING_PORT ^= out_bits & STEP_MASK;
-    _delay_us(step_delay);
+    delay_us(step_delay);
   }
   return;
 }