Complete HTML link extraction to cover all element attributes of type URI

[sebastian-nagel]

The HTML specs provide list of attributes including the required type. All attributes of type URI should be covered by the ExtractingParseObserver when links are extracted and added as "Links" to the WAT file. See

• HTML 4 attributes
• HTML 5 attributes
• (on stackoverflow) COMPLETE list of HTML tag attributes which have a URL value?

Several attributes are missing, e.g., "cite" for <q> and <blockquote>, or embedded elements introduced with HTML5 (<video>, <audio>).

This issue precedes #7 and #8, and should include a unit test.