make TapDance a module (#281)

* extract tapdance logic into a module

* clean out old tapdance code

* canonicalize key variable names

* split _process_tap_dance into td_pressed and td_released

* implement consistent argument order

* update documentation

* implement Module.process_key for key interception and modification

* fix tapdance realesing instead of pressing

* fix: default parameters in key handler

* cleanup holdtap

* add error handling to modules process_key

* fix: key released too late

Tapped keys didn't release on a "key released" event, but waited for a
timeout. Resulted in, for example, modifiers applying to keys after the
modifier was released.

* fix lint/formatting

* fix tap_time reference in modtap + minimal documentation

* fix lint

docs/modtap.md

@@ -4,7 +4,10 @@ added to the modules list.
 
 ```python
 from kmk.modules.modtap import ModTap
-keyboard.modules.append(ModTap())
+modtap = ModTap()
+# optional: set a custom tap timeout in ms
+# modtap.tap_time = 300
+keyboard.modules.append(modtap)
 ```
 
 ## Keycodes