Support evidence source

[mccalluc]

• Towards Link to evidence for each claim #169

If this much seems good, I think the next step would be to support multiple citations, and page number. I'd suggest embedding these in a string value, rather than complicating the schema, so it would look like:

name_source: 'example_1, p. 42; example_2, p. 404 (but that is probably a typo)'

• lists would split on ; (or new-line?)
• items would split on , (or :?)

(Normally, I'd say that explicitly encoding this in the schema is the way to go, but I'm already worried about the data-entry burden here, and that pushes me towards a delimited format.)