Cannot use my own textarea

https://github.com/andreypopp/react-textarea-autosize/blob/master/src/index.js#L82

Instead of rendering a `` this library should render children where the default props is a textarea with values.

This would give users who use this library more control if they wanted it with no downsides, it would not break existing users.

For example if I wanted to use Rebass TextArea then I would be able to specify my own text area element instead. At the moment this is impossible.