WIP: Provide join_html and join_to_html.

[kaj]

A possible solution to #85 .

Questions / things to test:

• Is join_html and join_to_html good names for the functions?
• Does join_html work with inline subtemplates when used in templates? It does not, and would be rather hard to fix at the moment. But inline templates with arguments would be a separate issue that would fix it.
• Can the return type be improved? I would like to return impl ToHtml, but currently both join_html and join_to_html returns Box<dyn ToHtml>, as a trait function apparently cannot return impl Trait. They could also return HtmlJoiner<...>, but then that class (which I consider an implementation detail) would have to be public, and the generic type arguments would be inconveniently long.
• Is the documentaion for join_html and join_to_html understandable?

[OskarPersson]

I think that both are good additions to the library, I really like the concept of join_html. However, I don't think destructuring tuples works in templates?

<!-- examples/simple/templates/hello_args.rs.html -->
@((greeting, who): &(&str, &str))
<h1>@greeting @who!</h1>

// examples/simple/src/main.rs
#[test]
fn test_hello_args() {
    assert_eq!(
        r2s(|o| hello_args_html(o, ("Hello", "World"))),
        "<h1>Hello World!</h1>\n",
    );
}

$ cargo t
warning: Template parse error in "templates/hello_args.rs.html":
warning:    1:@((greeting, who): &(&str, &str))
warning:        ^ expected formal argument

Also, would it be possible to have the template inline in join_html in addition to having the ability to have it as an external template?

[kaj]

Turns out the code as it is now has problem with types and/or lifetimes when actually using a template with join_html. Looking at how to fix it, may have to change the template function type and/or the ToHtml trait.