Add filter_map method to Subscription

[watsaig]

Just a handy way to go from Subscription<A> to Subscription<B> while removing any None values, using StreamExt::filter_map

The main use case is to work with existing subscription functions where one only cares about some values. For example:

fn subscription(&self) -> Subscription<Message> {
    window::events().filter_map(|(id, event)| match event {
        window::Event::Focused => Some(Message::WindowFocused(id)),
        window::Event::Closed => Some(Message::WindowClosed(id)),
        _ => None,
    })
}

Currently, doing this requires a no-op message variant.

[hecrj]

I haven't been a fan of treating subscriptions like composable units in the past; but I can't seem to argue that position reasonably today.

So, what the heck. Let's go ahead with this and see how it goes.

Thanks!