Extract ToC

[b-x-wu]

Hi!

This pull request adds the option to extract the ToC away from the root of the rest of the document. This way, if you pass in the relevant option to the plugin, instead of returning the entire document with its original root, it will return a document with only the ToC with its own new root.

The idea of this PR was to address #8, so let me know if this takes care of the request properly!

Cheers :)