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.

nuts_bolts.c

@@ -44,3 +44,10 @@ void delay_ms(uint16_t ms)
 {
   while ( ms-- ) { _delay_ms(1); }
 }
+
+// Delays variable defined microseconds. Compiler compatibility fix for _delay_us(),
+// which only accepts constants in future compiler releases.
+void delay_us(uint16_t us) 
+{
+  while ( us-- ) { _delay_us(1); }
+}