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.c

@@ -138,6 +138,8 @@ void report_feedback_message(uint8_t message_code)
     printPgmString(PSTR("Check Door")); break;
     case MESSAGE_PROGRAM_END:
     printPgmString(PSTR("Pgm End")); break;
+    case MESSAGE_RESTORE_DEFAULTS:
+    printPgmString(PSTR("Restoring defaults")); break;
   }
   printPgmString(PSTR("]\r\n"));
 }