Move fetcher away from the load_html_rendertree_fetcher, and create it at the calling function instead.

[jaytaph]

See discussion: #730 (comment)

I think this should be removed, instead callers should create their fetcher by themselves and give it load_html_rendertree_fetcher.

This function creates the fetcher, use it and returns it.. instead we probably want to have the calling function to create the fetcher for more flexibility:

let fetcher = fetcher::new();
let render_tree = load_html_rendertree(url, None, fetcher);