Acceleration independence installed. Initial re-work of planner for feedrate overrides.

NOTE: This push is a work-in-progress and there are known bugs that
need to be fixed, like homing acceleration being incompatible. Released
for testing. Settings will definitely be overwritten, as new settings
were needed.

- Acceleration independence installed in planner. Each axis can now
have different accelerations and Grbl will maximize the accelerations
depending on the direction its moving. Very useful for users like on
the ShapeOko with vastly different Z-axis properties.

- More planner optimizations and re-factoring. Slightly improved some
of the older calculations, but new acceleration calculations offset
these improvements. Overall no change in processing speed.

- Removed planner nominal length checks. It was arguable whether or not
this improved planner efficiency, especially in the worst case scenario
of arcs.

- Updated readme and changed to markdown format.

settings.h

@@ -29,7 +29,7 @@
 
 // Version of the EEPROM data. Will be used to migrate existing data from older versions of Grbl
 // when firmware is upgraded. Always stored in byte 0 of eeprom
-#define SETTINGS_VERSION 5
+#define SETTINGS_VERSION 50
 
 // Define bit flag masks for the boolean settings in settings.flag.
 #define BITFLAG_REPORT_INCHES      bit(0)
@@ -56,14 +56,14 @@
 
 // Global persistent settings (Stored from byte EEPROM_ADDR_GLOBAL onwards)
 typedef struct {
-  float steps_per_mm[3];
+  float steps_per_mm[N_AXIS];
   uint8_t microsteps;
   uint8_t pulse_microseconds;
   float default_feed_rate;
   float default_seek_rate;
   uint8_t invert_mask;
   float mm_per_arc_segment;
-  float acceleration;
+  float acceleration[N_AXIS];
   float junction_deviation;
   uint8_t flags;  // Contains default boolean settings
   uint8_t homing_dir_mask;