v8: Support of the className standard property (for styled-components or others)

[lmaze]

Hi there !

I'm using the styled-components lib in several projects and the DayPicker component doesn't seem to be able to be styled directly with it.

For example:

const StyledDayPicker = styled(DayPicker)`
  .caption_label {
    color: red;
  }
`;

From the styled-components documentation (https://styled-components.com/docs/advanced#styling-normal-react-components):

If you use the styled(MyComponent) notation and MyComponent does not render the passed-in className prop, then no styles will be applied. To avoid this issue, make sure your component attaches the passed-in className to a DOM node

It would be perfect if the DayPicker component supported the className property 😃 ❤️

In addition, the support of this property seems to be a good React practice to be able to style a component: https://reactjs.org/docs/faq-styling.html#how-do-i-add-css-classes-to-components