Support multiple attributes on a node

[cossssmin]

Using multiple attributes to transform contents doesn't work right now:

<div uppercase append=" - nice movie">fubar</div>

import content from 'posthtml-content'

const {html} = await posthtml(content({
    uppercase: content => content.toUpperCase(),
    append: (content, attribute) => content + attribute
  })).process(sourceHtml)

Not really sure what the result should be:

<div>FUBAR - NICE MOVIE</div>

or:

<div>FUBAR - nice movie</div>