Unable to use paper-slider full width

`paper-slider` has margins on div#sliderContainer[1] which is in the element's shadow DOM and not reachable by mixins or any other means. Users of the component should have some way to control the margin so that it can be used full width if desired.

[1]: https://github.com/PolymerElements/paper-slider/blob/master/paper-slider.html#L103