Integrate code to title or hide references section.

This automatically adds a title to the reference section pandoc-citeproc
produces, if there is not already a dangling section head. It takes
`reference-section-title` metadata as the reference section title. If this
metadata field is left blank, there is no automatic header.

It can also take a boolean metadtata `remove-ref-section` to remove the
reference section altogether. This can be useful for situations when you want
citations to be resolved, but don't want an extra few pages for references at
the end of the document.