Support intra-page URL fragment identifiers

@ebidel @addyosmani & co.:

Allows you to link directly to, e.g., an individual attribute or method in a specific doc page.
Supports `<core-doc-viewer>`s with multiple `sources=` set, or with a single `url=` set.
Maintains compatibility with using class names as the URL fragment identifier to switch between classes when multiple `sources=` is set.

Sample URLs:
- http://jeffposnick.github.io/core-doc-viewer/components/core-doc-viewer/demo.html#core-ajax.properties.xhrArgs
- http://jeffposnick.github.io/core-doc-viewer/components/core-doc-viewer/demo.html#core-icon.attributes
- http://jeffposnick.github.io/core-doc-viewer/components/core-doc-viewer/demo.html#core-icon.summary
- http://jeffposnick.github.io/core-doc-viewer/components/core-doc-viewer/demo.html#core-xhr
I don't think we need to add this.