Dynamic optional event handlers

## Specific Demand

Just like we can pass `Option` to element attributes when we want to pass dynamic optional attribute, it should be possible to do the same withe event handlers.

That way, this would work:
```rust
#[derive(Props)]
struct ComponentProps<'a> {
#[props(optional)]
onclick: Option>
}

fn Component<'a>(cx: Scope<'a, ComponentProps<'a>>) -> Element<'a> {
let onclick = cx.props.onclick.map(|v| move |e| {
v.call(e);
});

render!(
div {
onclick: onclick
}
)
}
```