Added restore settings defaults command.

- New restore setting defaults command. Only wipes ‘$$’ setting in
EEPROM and reloads them based on the defaults used when Grbl was
compiled. Used with a `$RST` command

NOTE: `$RST` is intentionally not listed in the Grbl ‘$’ help message.

grbl/report.h

@@ -69,6 +69,7 @@
 #define MESSAGE_DISABLED 5
 #define MESSAGE_SAFETY_DOOR_AJAR 6
 #define MESSAGE_PROGRAM_END 7
+#define MESSAGE_RESTORE_DEFAULTS 8
 
 // Prints system status messages.
 void report_status_message(uint8_t status_code);