title: Animate from current value with Polymer core-animation
body: <p>My Polymer application animates an object's <code>top</code> from an arbitrary position to <code>0</code>.  It's using Velocity to do that right now, since the way to animate from any current state to a new state is very obvious.  Is it possible (or even proper) to do with Polymer's <code>core-animation</code>?  All of the demos show fixed values at each keyframe.</p>