Add a simple format option for a state to convert brightness

[AmoebeLabs]

The Problem To Be Solved

Currently, one needs a derived_entity and some javascript to convert the brightness to a 0..100% range.

That can be done easier from a user point of view, yes?

Additional background:

Related Issues (if any)

(Optional): Suggested Solution

Support the 'format : brightness' option for an entity to just do that. Much easier!! Less YAML, and could be used for future additions!

(Optional): Alternative Solutions