Add support for setting refs on elements for DOM reference

[jstarry]

Proposal

Take inspiration from React Refs to get references to elements in the DOM

src/html/mod.rs

/// Wrapped Element reference for later use in Component lifecycle methods.
#[derive(PartialEq)]
pub struct ElementRef(RefCell<Option<Element>>);

impl Default for ElementRef {
    fn default() -> Self {
        ElementRef(RefCell::default())
    }
}

impl ElementRef {
    /// Get the wrapped Element reference if it exists
    pub fn get(&self) -> Option<Element> {
        self.0.borrow().clone()
    }

    /// Place an Element in a reference for later use
    pub fn set(&self, element: Element) {
        *self.0.borrow_mut() = Some(element);
    }
}

my_component.rs

#[derive(PartialEq, Properties)]
pub struct Props<T> {
    pub disabled: bool,
    pub select_ref: ElementRef,
}

// ...

fn mounted(&self) {
  if let Some(select) = self.props.select_ref.get() {
    // ...
  }
}

fn view(&self) -> Html<Self> {
  html! {
    <select ref={self.props.select_ref} disabled=self.props.disabled>
      // ...
    </select>
  }
}