Support nested transformations

[cossssmin]

Would be great to be able to do something like this:

<div uppercase>
  Some
  <br>
  <span reverse>text</span>
</div>

import content from 'posthtml-content'

const {html} = await posthtml(content({
    uppercase: content => content.toUpperCase(),
    reverse: content => content.split('').reverse().join('')
  })).process(sourceHtml)

... and get:

<div>
  SOME
  <br>
  <span>TXET</span>
</div>