SimpleUrlView for views identified by URL only

Fixes #9 

I think that we can probably reuse the `Condition` abstraction that exists in the synq library. It's a little different than what you have here since it uses Predicates and Matchers, but I think it would be nice to reuse that abstraction.

So you could have a `Condition<String>` and by default you could use a hamcrest matcher like, `matchCondition = HamcrestCondition.matchCallTo(getContext()::getCurrentUrl, equalTo(url));`. Then `isLoaded` just returns `matchCondition.isMet()`.

Or you could also just store a `Predicate<? super String>` and then the default can be `actualUrl -> actualUrl.equals(url)` or if we wanna use Hamcrest matchers (yay) we can set it `= Matchers.equalTo(url)::matches`. Then `isLoaded` returns `condition.test(getContext().getCurrentUrl());`.

Ultimately I think by reusing some existing structures we can avoid creating a new interface one way or another.