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/planner.c

@@ -320,7 +320,7 @@ uint8_t plan_check_full_buffer()
   // Adjust feed_rate value to mm/min depending on type of rate input (normal, inverse time, or rapids)
   // TODO: Need to distinguish a rapids vs feed move for overrides. Some flag of some sort.
   if (feed_rate < 0) { feed_rate = SOME_LARGE_VALUE; } // Scaled down to absolute max/rapids rate later
-  else if (invert_feed_rate) { feed_rate = block->millimeters/feed_rate; }
+  else if (invert_feed_rate) { feed_rate *= block->millimeters; }
   if (feed_rate < MINIMUM_FEED_RATE) { feed_rate = MINIMUM_FEED_RATE; } // Prevents step generation round-off condition.
 
   // Calculate the unit vector of the line move and the block maximum feed rate and acceleration scaled