Add URLHashDemo w/ window.onhashchange closure

[MaxDesiatov]

Resolves #284.

[MaxDesiatov]

@vi4m please check out the code in this PR. I imagine onpopstate would work similarly. The key bit here is that we need to manage lifetime of closures manually, because JavaScript doesn't have finalizers. Swift code can't be notified when JavaScript references to these closures are deallocated, so everything needs to be cleaned up manually. This is possible thanks to HashState being a reference type with deinit.

[vi4m]

Thanks! This code looks really easy on the eyes, and works perfectly.
I'm sure it will enrich documentation / help other early adopters as well 👍