Embed videos on paper page

It would be nice to embed videos on the paper pages. This shouldn't be hard to implement. It would require:

1. Modifying the paper page template, hugo/layouts/papers/single.html, to embed the video
2. Maybe writing a custom field in the generated YAML that is used by Hugo to build the Anthology. This would be done by creating a field, say video_embed_link, in the from_xml() function in bin/anthology/papers.py. (This file is used to load papers from the XML file, and all of its attrib attributes are dumped to the YAML).

We link to videos on Vimeo and Slideslives, which likely have different embedding code blocks, which is the reason for point 2 above.