Add RIS format to enable exporting citations to RIS.

An initial implementation of RIS export format.

Possible issues:
* Use of <br /> tags for line breaks
* Doesn't attempt to exhaustively map all the fields of all the RIS types to all the possible CSL variables.

This is a minimally viable CSL for our needs at Faithlife for use in Logos Bible Software, but we'd prefer to have this integrated upstream so the community can improve it and others can benefit.