Added coolant control (M7*, M8, M9). Mist control can be enabled via config.h.

- Added coolant control! Flood control (M8) functions on analog pin 0.
Mist control (M7) is compile-time optional and is on analog pin 1. (Use
only if you have multiple coolants on your system). Based on work by
@openpnp.

- Fixed some variable assignments in spindle control.

main.c

@@ -26,6 +26,7 @@
 #include "nuts_bolts.h"
 #include "stepper.h"
 #include "spindle_control.h"
+#include "coolant_control.h"
 #include "motion_control.h"
 #include "gcode.h"
 #include "protocol.h"
@@ -73,6 +74,7 @@ int main(void)
       plan_init(); // Clear block buffer and planner variables
       gc_init(); // Set g-code parser to default state
       spindle_init();
+      coolant_init();
       limits_init();
       st_reset(); // Clear stepper subsystem variables.