New report module. 6 persistent work coordinates. New G-codes and settings. README and minor bug updates

(NOTE: This push is likely buggy so proceed with caution. Just
uploading to let people know where we're going.)

- New report.c module. Moved all feedback functions into this module to
centralize these processes. Includes realtime status reports, status
messages, feedback messages.

- Official support 6 work coordinate systems (G54-G59), which are
persistently held in EEPROM memory.

- New g-code support: G28.1, G30.1 stores current machine position as a
home position into EEPROM. G10 L20 Px stores current machine position
into work coordinates without needing to explicitly send XYZ words.

- Homing performed with '$H' command. G28/G30 no longer start the
homing cycle. This is how it's supposed to be.

- New settings: Stepper enable invert and n_arc correction installed.

- Updated and changed up some limits and homing functionality. Pull-off
travel will now move after the homing cycle regardless of hard limits
enabled. Fixed direction of pull-off travel (went wrong way).

- Started on designing an internal Grbl command protocol based on the
'$' settings letter. Commands with non numeric characters after '$'
will perform switch commands, homing cycle, jogging, printing
paramters, etc. Much more to do here.

- Updated README to reflect all of the new features.

serial.c

@@ -166,7 +166,7 @@ ISR(USART_RX_vect)
     case CMD_FEED_HOLD:     sys.execute |= EXEC_FEED_HOLD; break; // Set as true
     case CMD_RESET:
       // Immediately force stepper and spindle subsystem idle at an interrupt level.
-      if (!(sys.execute & EXEC_RESET)) { mc_alarm(); } // Stop only first time.
+      mc_alarm();
       sys.execute |= EXEC_RESET; // Set as true
       break;
     default: // Write character to buffer