Allow the section selectors for comment sections to be configured

Moves section selector into `index.js` config. fixes #31.

Related changes to better support other section types (mainly `img`):
* Section selection only returns top level elements from the article.
* Filter uses `:empty` instead of `isEmpty(x.text())`. This fixes the case `<p><img /></p>` but now include cases like `<p><input /></p>`
* Added `:has(img)` to default selector set.