Support inlining via data-url

I stumbled upon this while searching for a solution to a particular issue of mine - Its connecting the base html element with the way cloudflare workers work and with the manifest.json#start_url attribute required for installation of PWAs.

I'm probably gonna find another solution, I mostly wanted to document this here as something that might be helpful in some situations. If its easy to implement it'd be great, otherwise its fine with me if this doesn't happen.

Data-Urls can be used to encode some things in the html document, that otherwise can't be inlined.

It'd be nice if these were supported with a data-* attribute by trunk.