Custom button css class

[JannikZed]

We would like to overwrite the button class in order to use our standard styling. Right now, we need to create a redundant section in our CSS file.

[artdevgame]

@JannikZed It looks like you can do that by passing buttonClassName: https://github.com/gndx/react-mailchimp-form/blob/master/src/index.js#L104