Add more values for controlling animations

[ruyadorno]

The way the animations is currently handled right now is very limiting, barely only working for the opacity transition scenario.

We should add one more step to make it easier to custom at the very least:

• startValue: Initial value at the start of transition anim
• visibleValue: The value the object property should have when visible
• endValue: Value the property should have at the end of exit anim

This will allow us to define different animations for intro and exit states. That should be properly documented too.