ripple: option to keep ripple persistent

[devversion]

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.

let ripple = ripple.launch(0, 0, { persist: true });
// Manually fade-out the persistent ripple
ripple.fadeOut();

Related to #2520

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