Add basic template macro

[NthTensor]

Adds a very basic declarative template system, with rudimentary incrementalization.

fn hello_to(name: String, party_time: bool) -> Template {
    template! {
        root: { Node::default() } [
            greetings: { TextSpan::new("Hello") };
            name: {
                if !party_time {
                    (TextSpan::new(name), TextColor::default())
                } else {
                    (TextSpan::new(format!("{}!!!!!", name)), TextColor(css::HOT_PINK.into()))
                }
            };
        ];
    }
}

See the included example for a more complete indication of how it works. Caveat emptor!

[alice-i-cecile]

Can you add a note to the RELEASES.md and bump the version number? Let's ship this :)

[NthTensor]

Sweet! I'll let you press the button. (Thanks for the corrections, it looks like maybe my editor's spellcheck has broken).