don't close img tag for html5

knockoff emits XHTML, but in HTML5 `img` is a [void element](http://www.w3.org/html/wg/drafts/html/master/syntax.html#void-elements).

(I first thought this was causing some previewing issue when looking at "Additional resources" page in Pamflet's pamflet, but it seems to be more to do with svg than img tag since I was able to display png fine.)