Provide a way to inject links/metadata in the head and script in the footer

[amoutonbrady]

Often time you want to have a simple setup that's already made, especially for the style. Or you want to reproduce a very specific case with weird metadata.

Maybe this could be part of the settings page mentioned in #15 but I'd like to have stupid simple way of just writing links, metadata and script to the iframe.

This would probably be just a text area for head and one for footer than we would then read and inject in the iframe code.