Add configurable harvester options.

Summary:
 - Add configurable metadata which can be included in the harvested
 <mws:data/>.
 - Add configurations for ZBL and Arxiv.
 - Closes #25.

Test Plan:
bin/docs2harvest -c config/arxiv-harvester.json test/data/astro-ph0001197.html
bin/docs2harvest -c config/zbl-harvester.json test/data/zbl4138077.xhtml

Reviewers: radu.hambasan

Differential Revision: http://phab.code4fun.de/D65