Allow to replace an element instead of append to body for web rendering

[traxys]

In case you want to have the application at the start of the page and some other content after it just appending to the body won't work

With this change you can add a <div id="iced_root"></div> and it will be replaced by the canvas

[13r0ck]

How cool would it be to have an Elm lang package that allows for Iced embeds using this! Elm could handle all the SEO, pages, general web stuff, and Iced could be for the parts that don't need SEO and/or rendering of more complicated features.