Support custom component (e.g. textarea)

[kevinresol]

Also allow ref to the actual DOM element (via inputRef)

[LeoEatle]

Good idea about this one. But I'm considering it strange that the component name is react-composition-input but we create a textarea.
How about creating a HOC component so we can provide attributes and callback for handling child component? But in this way it is not easy to use.

Maybe I will merge it in a few weeks.

[kevinresol]

"input" doesn't necessarily refer to the DOM element. Generally speaking, textarea is also an input.