Consider adding support for loading user selectable css resources per element

[sorvell]

Support for customizing styling in custom elements using ShadowDom is still evolving at the spec level.

As a workaround until this settles down, we could consider adding a system for customizing the location of the css resources a custom element needs.

Perhaps something like:

<script>
  Polymer.resources({
    'polymer-ui-toolbar': 'http://some-base-path'
  });
</script>

Then in polymer-ui-toolbar a specific resource that does not have an absolute url could be path adjusted by this base path:

<link rel="stylesheet" href="styles.css">

loads from http://some-base-path/styles.css.

[sorvell]

We've decided against this.