[Request] CustomAtSpeed function

[MrVizious]

Right now, tweens like Position have a PositionAtSpeed version that admits a value for the speed instead of the duration. When trying to tween between two colors, the documentation shows that Tween.Custom should be used, and it does its job fantastic. However, I miss a CustomAtSpeed function so I don't have to make a calculation everytime I want to tween between those values at a specified speed instead of with a fixed duration.

Really good work with the project, congratulations!