add support for specifying html

[rukai]

Winit developers have brought up the desire to provide a full HTML file to cargo-run-wasm.
They want this as they need to provide a "javascript user" focused example, so having to use web_sys to write the html/javascript is no good.
See rust-windowing/winit#2816 and rust-windowing/winit#2852

I think its pretty clear that we need this feature, but I'm not sure yet how it should be designed.

Some competing ideas:

• turn run_wasm_with_css into a builder struct that can also set the html body
• Introduce a separate run_wasm_with_html_body function that inserts an html body into the html file
• Introduce a separate run_wasm_with_html function that takes a full html file and performs some templating on it to insert the wasm script.

[Vrixyz]

Am I understanding correctly that #40 laid out the foundation so we can hook to RunWasm to add an html parameter capability ?