Controllable boolean attributes

[simonchatts]

For boolean attributes such as hidden or checked, it would be handy to be able to control the presence or absence via a boolean expression.

One way to do this is what Yew does: require a boolean value in the template! (as is required today) but include the attribute if-and-only-if the value is true.

As an example, this would shrink code like this:

    template! {
        (if *is_empty.get() {
            template! {
                button(type="submit", class="btn btn-outline-primary", disabled=true) { "Submit" }
            }
        } else {
            template! {
                button(type="submit", class="btn btn-outline-primary") { "Submit" }
            }
        })
    }

down to this:

    template! {
        button(type="submit", class="btn btn-outline-primary", disabled=*is_empty.get()) { "Submit" }
    }