Basic support for <object>

[SimonSapin]

Needed for #1366 (Acid2). CC @recrack

The <object> element is kind of a mix of <img> and <iframe>. As a first step (which should be enough for Acid2), I suggest we treat it like <img> with two differences:

• The resource’s URL is given by the data attribute instead of src,
• and the fallback (used when the resource fails to load) is given by the element’s content (ie. until the </object> closing tag) instead of the alt attribute. (As with <img alt>, the fallback is not shown at all when the resource does load.)

[recrack]

@SimonSapin Thanks for the suggestion!

[pcwalton]

Fixed.