Selector: Webkit broke sibling of filtered id selection

Fixes #290

Increases minified+gzipped size by 30 bytes... ridiculous. Does anyone see room for further improvement?
I actually did try that, but couldn't get it to a net benefit after the necessary rearranging of `id='d\f]'`, which is no longer _guaranteed_ valid (i.e., unique) once we're in the document. However, I just gave it another shot and pulled it off; I'll push here after testing with iOS 8. Thanks!