@demo path should be relative to source file, not component page.

[MaKleSoft]

Currently it seems that iron-component-page expects the link provided through the @demo tag is relative to the component page. This is problematic for several reasons:

• Its counterintuitive. Data point of one, but I would expect the link to be relative to the source file.
• The element can't be included in multiple component pages at once.
• When analyzing the file individually, one has to know the location of the component page in order to determine the location of the demo page. In fact, without the existence of a component page, the demo path is not well-defined. This is relevant for other analytics tools like polylint.

To ensure backward compatibility, iron-component-page could try to resolve the demo path both relative to the component page and relative to the source file.