Update CSSProperties.json

Added background-blend-mode CSS property. This is in shipping versions of Firefox, Chrome and Opera and Under Consideration for IE. It's an Editor's Draft at W3C and a neat feature so I think worthy of being added to Brackets.

Reference:
http://dev.w3.org/fxtf/compositing-1/
http://docs.webplatform.org/wiki/css/properties/background-blend-mode