Add common energy module and deprecate device emeter attributes (#976)

Consolidates logic for energy monitoring across smart and iot devices.
Deprecates emeter attributes in favour of common names.

kasa/interfaces/__init__.py

@@ -1,5 +1,6 @@
 """Package for interfaces."""
 
+from .energy import Energy
 from .fan import Fan
 from .led import Led
 from .light import Light, LightState
@@ -8,6 +9,7 @@ from .lightpreset import LightPreset
 
 __all__ = [
     "Fan",
+    "Energy",
     "Led",
     "Light",
     "LightEffect",