Auto auto-off module for smartdevice (#760)

Adds auto-off implementation. The feature stays enabled after the timer runs out, and it will start the countdown if the device is turned on again without explicitly disabling it.

New features:
* Switch to select if enabled: `Auto off enabled (auto_off_enabled): False`
* Setting to change the delay: `Auto off minutes (auto_off_minutes): 222`
* If timer is active, datetime object when the device gets turned off: `Auto off at (auto_off_at): None`

kasa/smart/modules/__init__.py

@@ -1,4 +1,5 @@
 """Modules for SMART devices."""
+from .autooffmodule import AutoOffModule
 from .childdevicemodule import ChildDeviceModule
 from .cloudmodule import CloudModule
 from .devicemodule import DeviceModule
@@ -12,6 +13,7 @@ __all__ = [
     "EnergyModule",
     "DeviceModule",
     "ChildDeviceModule",
+    "AutoOffModule",
     "LedModule",
     "CloudModule",
     "LightTransitionModule",