Add backgroundColor/color as an animatable keyframe properties

[Austin Malerba]

I originally avoided adding backgroundColor as an animatable keyframe property because it falls into a category of less efficient properties to animate.

Changing backgroundColor causes the browser to repaint and recomposite the ui, whereas changes to transform/opacity only cause the browser to recomposite the ui.

However, given the demand for animating backgroundColor/color, i think it's worth allowing these as exceptions.