Added block delete, opt stop, single block mode. New parser state and parameter feedback. Overhauled '$' command.

NOTE: Another incremental update. Likely buggy, still a ways to go
before everything is installed, such as startup blocks.

- Changed the '$' command to print help. '$$' now prints Grbl settings.
The help now instructs the user of runtime commands, switch toggling,
homing, etc. Jogging will be added to these in v0.9.

- Added switches: block delete, opt stop, and single block mode.

- Now can print the g-code parser state and persistent parameters
(coord sys) to view what Grbl has internally.

- Made the gc struct in the g-code parser global to be able to print
the states. Also moved coordinate system tracking from sys to gc struct.

- Changed up the welcome flag and updated version to v0.8c.

- Removed spindle speed from gcode parser. Not used.

spindle_control.c

@@ -43,7 +43,7 @@ void spindle_stop()
   SPINDLE_ENABLE_PORT &= ~(1<<SPINDLE_ENABLE_BIT);
 }
 
-void spindle_run(int8_t direction, uint16_t rpm) 
+void spindle_run(int8_t direction) //, uint16_t rpm) 
 {
   if (direction != current_direction) {
     plan_synchronize();