Fix warning in Home Assistant log.

A sensor can now be selected as Remaining Duration Entity next to input numbers.

Fix issue if price is exactly 0.
Fix issue introduced with release 1.2.1

Changed behavior if duration entity:

Entity represents the remaing duration now. So, if the duration entity state changes while the configured interval is active, the state value represents the remaining duration from now on.

Added support for dynamic duration using an input entity.

Initial release