Usage: brightness(amount)
This filter increases or decreases the image brightness.
amount--100 to 100- The amount (in %) to change the image brightness. Positive numbers make the image brighter and negative numbers make the image darker.
http://localhost:8888/unsafe/filters:brightness(40)/https%3A%2F%2Fgithub.com%2Fthumbor%2Fthumbor%2Fraw%2Fmaster%2Fexample.jpg
