insert a preamble before pandoc-citeproc's bibliography

 pandoc-citeproc-preamble is a JSON filter for Pandoc which inserts a
 preamble before the output that the pandoc-citeproc filter appends to
 the document. This preamble might include a heading
 (e.g. "Bibliography") and raw markup to format the bibliography for
 the output format.
 .
 Since pandoc-citeproc doesn't provide any facility to add formatting
 control code to its output, pandoc-citeproc-preamble is necessary to
 avoid the user being forced to add their control code to the end of
 their input files, thereby losing the input file's agnosticity with
 regard to output format.