Runs `scrolled` if content changes (i.e. async load)

Fixes issue following PR #15 being merged, specifically that the initial default-cloaking will result in content loaded asynchronously (i.e. after the view is inserted) also being cloaked, even if in view, until a scroll event is fired. This fix also fires `scrolled` if `content.@each` is updated.

In lieu of test cases, I've also added `demos/async.html` where this fix can be verified (I'm OK with removing this if not needed – the only difference from the main demo page is how the model is set).