Support converting elements to HTML

[chipsenkbeil]

This should be fairly straightforward. Not sure if I'd want to use typed-html given it is not maintained, but having some type guarantees would be nice and it can still produce string output.

Idea would be to have some trait that all elements implement:

pub trait ToHtml {
    fn to_html_string(&self) -> String;
}

Not sure if it actually needs to be a trait. I guess for supporting trait objects if we needed that, this trait would be helpful.

[chipsenkbeil]

Maybe leverage html5ever crate to parse and write back out the HTML produced? Need to see if this actually formats the html. See example

[chipsenkbeil]

If we don't care about formatting, can also just use htmlescape to do proper sanitization of HTML generated.

[chipsenkbeil]

Actually, use https://github.com/rust-ammonia/ammonia as it seems to be more modern, well-supported, and faster.

[chipsenkbeil]

Should provide some way to keep track of indentation