Rechargeable battery sensor

Added

• Added battery_percent_rechargeable sensor to provide a more realistic
  battery level estimation for rechargeable batteries (e.g. NiMH).
• The new sensor derives remaining charge from pack voltage and is intended
  for monitoring and automations when using rechargeable cells.
• Existing battery_percent sensor continues to reflect the vendor/device
  battery percentage logic.

Changed

• Clarified separation between vendor-reported battery percentage and
  voltage-based rechargeable battery estimation.