Addition of new binary sensor class 'plug' (#10336)

* Addition of new binary sensor class 'plug'

* use term "unplugged"

* add the entry to the right place

homeassistant/components/binary_sensor/__init__.py

@@ -30,6 +30,7 @@
     'moving',        # On means moving, Off means stopped
     'occupancy',     # On means occupied, Off means not occupied
     'opening',       # Door, window, etc.
+    'plug',          # On means plugged in, Off means unplugged
     'power',         # Power, over-current, etc
     'safety',        # Generic on=unsafe, off=safe
     'smoke',         # Smoke detector