Pass className to child components so that it may work with styled-components

[beepsoft]

Hi!

I just found your project and I really like the audio visualization while recording the audio!

One thing that would make styling AudioReactRecorder easier if you added to the

<div className='audio-react-recorder'>

the this.props.className inherited from the parent component. Something like this:

let className = 'audio-react-recorder'
if (this.props.className) {
    className += " "+this.props.className
}
...
<div className={className}>

This would make AudioReactRecorder work with https://styled-components.com/