fix TODO for VNode::apply for VNode::VRef variant

[vitiral]

Here is the current code:

    /// Virtual rendering for the node. It uses parent node and existend children (virtual and DOM)
    /// to check the difference and apply patches to the actual DOM represenatation.
    fn apply(
        &mut self,
        parent: &Node,
        precursor: Option<&Node>,
        opposite: Option<VNode<Self::Context, Self::Component>>,
        env: ScopeEnv<Self::Context, Self::Component>,
    ) -> Option<Node> {
        match *self {
            VNode::VTag(ref mut vtag) => vtag.apply(parent, precursor, opposite, env),
            VNode::VText(ref mut vtext) => vtext.apply(parent, precursor, opposite, env),
            VNode::VComp(ref mut vcomp) => vcomp.apply(parent, precursor, opposite, env),
            VNode::VList(ref mut vlist) => vlist.apply(parent, precursor, opposite, env),
            VNode::VRef(_) => {
                // TODO use it for rendering any tag
                unimplemented!("node can't be rendered now");
            }
        }
    }

This is making it so that we can't shim arbitrary javascript nodes into yew:

koute/stdweb#196 (comment)

This could offer preliminary support for #143

[vitiral]

I think I'm missing something -- how are we communicating to javascript that our node is doing anything? I feel like the apply method is doing it with the parent...

[vitiral]

This is related to #189 and makes me at least convinced that it is non-trivial.