How to handle multiple URLs in schema?

[ejhumphrey]

per discussion in #13, what if there are mirrors? or multiple related sites? should url be an array? or do we need a separate mirrors or additional_urls field?

[ejhumphrey]

for what it's worth, I don't think anyone/thing is consuming the raw YAML file directly outside the render_datasets.py script, so a semi-serious schema change won't have cascading effects.