Add support for iframes

Usage:

Set `iframe: true` in model's YAML Frontmatter to embed that model in iframe.
Useful for 'leaky' stylesheets (e.g. Bootstrap or external imports) that are difficult
to scope under module (include body and other default styles that may affect Glypotheque.

Additionally use `pretty: true` on the model to force reformat HTML remotely fetched HTML