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.

planner.h

@@ -41,8 +41,8 @@ typedef struct {
   float entry_speed_sqr;             // Entry speed at previous-current block junction in mm/min
   float max_entry_speed_sqr;         // Maximum allowable junction entry speed in mm/min
   float millimeters;                 // The total travel of this block in mm
+  float acceleration;
   uint8_t recalculate_flag;          // Planner flag to recalculate trapezoids on entry junction
-  uint8_t nominal_length_flag;       // Planner flag for nominal speed always reached
 
   // Settings for the trapezoid generator
   uint32_t initial_rate;              // The step rate at start of block