New compile options and inverse time bug fix.

- Apparently inverse time motion were not working for quite some time.
Goes to show how many people actually use it. The calculation was bad
and is now fixed in this update. It should now work correctly.

- `;` comment type is now supported. This is standard on LinuxCNC and
common on 3d printers. It was previously not supported due to not
existing in the NIST standard, which is out-dated.

- New compile-option to ECHO the line received. This should help users
experiencing very weird problems and help diagnose if there is
something amiss in the communication to Grbl.

- New compile-option to use the spindle direction pin D13 as a spindle
enable pin with PWM spindle speed on D11. This feature has been
requested often from the laser cutter community. Since spindle
direction isn’t really of much use, it seemed like good good trade.
Note that M4 spindle enable counter-clock-wise support is removed for
obvious reasons, while M3 and M5 still work.

grbl/report.c

@@ -400,6 +400,15 @@ void report_build_info(char *line)
 }
 
 
+// Prints the character string line Grbl has received from the user, which has been pre-parsed,
+// and has been sent into protocol_execute_line() routine to be executed by Grbl.
+void report_echo_line_received(char *line)
+{
+  printPgmString(PSTR("[echo: ")); printString(line);
+  printPgmString(PSTR("]\r\n"));
+}
+
+
  // Prints real-time data. This function grabs a real-time snapshot of the stepper subprogram 
  // and the actual location of the CNC machine. Users may change the following function to their
  // specific needs, but the desired real-time data report must be as short as possible. This is