Teach parser to interpret image markup (wikitext, 9f49894)

Given wikitext input like:

{{foo.png}}

will emit:

<img src="foo.png" alt="foo.png" />

The "alt" attribute is added because it is required for XHTML validity. In the future I may add a support for overriding the attribute content, perhaps with a syntax like this:

{{foo.png|a picture of foo}}

Signed-off-by: Greg Hurrell <greg@hurrell.net>