value and immediate-value attributes

[MeloJR]

I found a problem using `paper-slider. Suppose I have one component, such as an image-resizer, which has a zoom property I want to bind to the value of my paper-slider. Something like that:

<image-resizer zoom="{{_zoom}}"></image-resizer>
<paper-slider value="{{_zoom}}"></paper-slider>

The problem with this code is that I have no immediate feedback when I move the slider, and so I have to bind to immediate-value if I want immediate-feedback:

<image-resizer zoom="{{_zoom}}"></image-resizer>
<paper-slider immediate-value="{{_zoom}}"></paper-slider>

Now, if I wanted to set the value of _zoom manually (or by an action in image-resizer, such as zoom with 2 fingers) the value of paper-slider will not change, because immediate-value is read-only. I don't know how to fix that in a clean way, how would you tackle this problem?

From my perspective, it would be better if paper-slider only had the value attribute and, in case immediate feedback is needed, the binding could be changed to update in the immediate-value-change event:

<image-resizer zoom="{{_zoom}}"></image-resizer>
<paper-slider value="{{_zoom::immediate-value-changed}}"></paper-slider>

[blasten]

This sounds very reasonable, feel free to send a PR. I like <paper-slider value="{{_zoom::immediate-value-changed}}"></paper-slider>, but it would be a breaking change. Alternatively, immediate-value could be editable. Issue related: https://github.com/PolymerElements/paper-slider/issues/32