Leader key implementation (#326)

* implements leader key for planck experimental

* allows override of leader timeout

* adds ability to use the leader key in seq

* fixes leader keycode

* adds chording prototype

* fixes keycode detection

* moves music mode to quantum.c

* disables chording by default

* updates process_action functions to return bool

quantum/quantum.mk

@@ -1,7 +1,8 @@
 QUANTUM_DIR = quantum
 
 # # project specific files
-SRC += $(QUANTUM_DIR)/keymap_common.c \
+SRC += $(QUANTUM_DIR)/quantum.c \
+	$(QUANTUM_DIR)/keymap_common.c \
 	$(QUANTUM_DIR)/led.c
 
 # ifdef KEYMAP_FILE