Add LeaderMode.TIMEOUT (QMK default Leader mode)

This allows leader sequences to "time out" rather than requiring an
Enter keypress to end.

This also rolls back some unnecessary changes from #72 to the matrix
scanner for performance reasons.

In theory we can use this in the future for Tap Dance support (#40)

Resolves #1
Resolves #37

kmk/consts.py

@@ -150,7 +150,7 @@ class UnicodeModes:
 
 
 class LeaderMode:
-    DEFAULT = 0
-    DEFAULT_ACTIVE = 1
+    TIMEOUT = 0
+    TIMEOUT_ACTIVE = 1
     ENTER = 2
     ENTER_ACTIVE = 3