Add skip_brightness_increases conf option

[timstallmann]

I'm finding this useful for my particular use case:

• I mostly have lights off during the day and on at night
• I have an automation to disable adaptive lighting when the sun is up, so that I have full manual control of lights during that time
• In the evenings, I'll (manually or via automation) turn some lights on dim for ambiance
• I'd still like adaptive lighting to gradually dim everything over the evening

This PR adds a config option to disable brightness increases, which just skips sending any command where the current entity brightness is less than the target brightness.