Move dimmer support to its own class (#34)

* Move dimmer support to its own class

SmartDimmer extends SmartPlug with brightness settings.
This will make the API of SmartPlug less confusing and will
make it simpler to downstream users to act with dimmers.

Fixes #33

* Lint & make tests pass

* Fix rebase after cache and emeter cleanups, hopefully everything went smoothly..

* oopsie, has_emeter was mistakenly included in smartplug

kasa/smartdevice.py

@@ -29,6 +29,7 @@ class DeviceType(Enum):
     Plug = 1
     Bulb = 2
     Strip = 3
+    Dimmer = 4
     Unknown = -1
 
 
@@ -583,6 +584,11 @@ class SmartDevice:
         """Return True if the device is a strip."""
         return self._device_type == DeviceType.Strip
 
+    @property
+    def is_dimmer(self) -> bool:
+        """Return True if the device is a dimmer."""
+        return self._device_type == DeviceType.Dimmer
+
     @property
     def is_dimmable(self) -> bool:
         """Return  True if the device is dimmable."""