Add ColorTweener.Framerate

[badslug]

The default frame rate should be 60fps but there should be a property on ColorTweener to change it from the default if desired. I don't think we need to add it as a parameter to NewColorTweener as the vast majority of the time, the default frame rate should be fine. Obviously once Start() is called, the frame rate should be locked into whatever was set on the ColorTweener - changing the value after Start() is called has no effect on the currently running tweener.