Add prop innerRef to access textarea DOM node

[FrancescoCioria]

requirements

As gaeron said here, sooner or later ReactDOM.findDOMNode will be deprecated and it will be apt to each component to decide if the user will be able or not to access the DOM node by its standard prop API.

We should add a prop innerRef: t.maybe(t.Function) that is called with the ref to the DOM node if it exists.

specs

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misc

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