Include support for module documentation

With changes to `<iron-doc-viewer>` to include a new `<iron-doc-module>`, `<iron-component-page>` needs to be updated to support listing module files in the navigation.

See https://github.com/PolymerElements/iron-doc-viewer/issues/179, https://github.com/PolymerElements/iron-doc-viewer/pull/175.