Add className in props
Hi,

Could you add className to props of those react color components? So I can use them like this:

```javascript
import React from 'react';
import { SketchPicker } from 'react-color';

class Component extends React.Component {

  render() {
    return <SketchPicker className="my-sketch-picker" />;
  }
}
```

Without it StyledComponents cannot work: [https://github.com/styled-components/styled-components/issues/1086](https://github.com/styled-components/styled-components/issues/1086)

And a custom className may also make the work much easier for a lot of people.

Thanks for considering it.

Regards,
Searene