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/config.h

@@ -251,6 +251,22 @@
 // spindle RPM output lower than this value will be set to this value.
 // #define MINIMUM_SPINDLE_PWM 5 // Default disabled. Uncomment to enable. Integer (0-255)
 
+// By default on a 328p(Uno), Grbl combines the variable spindle PWM and the enable into one pin to help 
+// preserve I/O pins. For certain setups, these may need to be separate pins. This configure option uses
+// the spindle direction pin(D13) as a separate spindle enable pin along with spindle speed PWM on pin D11. 
+// NOTE: This configure option only works with VARIABLE_SPINDLE enabled and a 328p processor (Uno). 
+// NOTE: With no direction pin, the spindle clockwise M4 g-code command will be removed. M3 and M5 still work.
+// #define USE_SPINDLE_DIR_AS_ENABLE_PIN // Default disabled. Uncomment to enable.
+
+// With this enabled, Grbl sends back an echo of the line it has received, which has been pre-parsed (spaces
+// removed, capitalized letters, no comments) and is to be immediately executed by Grbl. Echoes will not be 
+// sent upon a line buffer overflow, but should for all normal lines sent to Grbl. For example, if a user 
+// sendss the line 'g1 x1.032 y2.45 (test comment)', Grbl will echo back in the form '[echo: G1X1.032Y2.45]'.
+// NOTE: Only use this for debugging purposes!! When echoing, this takes up valuable resources and can effect
+// performance. If absolutely needed for normal operation, the serial write buffer should be greatly increased
+// to help minimize transmission waiting within the serial write protocol.
+// #define REPORT_ECHO_LINE_RECEIVED // Default disabled. Uncomment to enable.
+
 // Minimum planner junction speed. Sets the default minimum junction speed the planner plans to at
 // every buffer block junction, except for starting from rest and end of the buffer, which are always
 // zero. This value controls how fast the machine moves through junctions with no regard for acceleration
@@ -261,7 +277,8 @@
 
 // Sets the minimum feed rate the planner will allow. Any value below it will be set to this minimum
 // value. This also ensures that a planned motion always completes and accounts for any floating-point
-// round-off errors. A lower value than 1.0 mm/min may work in some cases, but we don't recommend it.
+// round-off errors. Although not recommended, a lower value than 1.0 mm/min will likely work in smaller
+// machines, perhaps to 0.1mm/min, but your success may vary based on multiple factors.
 #define MINIMUM_FEED_RATE 1.0 // (mm/min)
 
 // Number of arc generation iterations by small angle approximation before exact arc trajectory 
@@ -353,14 +370,20 @@
 // #define HARD_LIMIT_FORCE_STATE_CHECK // Default disabled. Uncomment to enable.
 
 
-// ---------------------------------------------------------------------------------------
-
-// TODO: Install compile-time option to send numeric status codes rather than strings.
-
 // ---------------------------------------------------------------------------------------
 // COMPILE-TIME ERROR CHECKING OF DEFINE VALUES:
 
+#ifndef HOMING_CYCLE_0
+  #error "Required HOMING_CYCLE_0 not defined."
+#endif
 
+#if defined(USE_SPINDLE_DIR_AS_ENABLE_PIN) && !defined(VARIABLE_SPINDLE)
+  #error "USE_SPINDLE_DIR_AS_ENABLE_PIN may only be used with VARIABLE_SPINDLE enabled"
+#endif
+
+#if defined(USE_SPINDLE_DIR_AS_ENABLE_PIN) && !defined(CPU_MAP_ATMEGA328P)
+  #error "USE_SPINDLE_DIR_AS_ENABLE_PIN may only be used with a 328p processor"
+#endif
 
 // ---------------------------------------------------------------------------------------