ripple: option to keep ripple persistent

As @crisbeto mentioned per chat, he needs a way for ripples to persist until they will be programmatically hidden.

We thought about another option for the `RippleConfig` and also about exposing an object for the ripple.

```ts
let ripple = ripple.launch(0, 0, { persist: true });
// Manually fade-out the persistent ripple
ripple.fadeOut();
```

Related to #2520 