Revamp API to cleanly separate configuration and effects

This is a more philosophical question about the API of this lib

Currently, the component depends on its effects propagating upwards by a lifecycle-ish `onChange` method, which usually triggers a change in state of the parent component / update store etc. However, this approach merges together the _configuration_ of the component (`scrollDelay`, `intervalDelay` etc), with its _effects_ (`onChange`), which makes it slightly confusing to grok IMO.

Inspired by React Router v4, I propose the following API

```jsx
const MyVisibilitySensor = createSensor({
delayInterval : 200,
// other opts
});

const App = () => (

{
({ isVisible }) => (
isVisible ? :
)
}

);
```

This separates concerns very nicely, and also avoids the overhead of having a situation where we have a hierarchy of parent component -> visibility sensor, and the `onChange` callback triggers a re-render from the parent component downwards. It helps avoid one layer of lifting state up, and is easier to read IMO.

Thoughts?