[docs] Styled API Example (5 lines)

[mssngr]

Was implementing the styled-components API example, but I needed to be able to set classes to the various sub classes in many of material-ui's components (such as paper in the Drawer component.) I then realized the API example could be written much more simply while giving it that extra bit of functionality. Decided to do a PR, since it worked so well for me.

• Reduces styled-components API example down to 5 lines
• Adds classes functionality to the styled-components API example
• Updates the documentation to reflect these changes

// Yes, 5 lines of code, it's all you need.
const styled = Component => (style, options) => {
  const StyledComponent = props => <Component {...props} />;
  const styles = typeof style === 'function' ? theme => ({ ...style(theme) }) : { ...style };
  return withStyles(styles, options)(StyledComponent);
};

[oliviertassinari]

@mssngr Thanks for the pull-request.

I then realized the API example could be written much more simply while giving it that extra bit of functionality

But it's no longer following the styled-components API. I mean, it's introducing an important difference. Also, this doesn't work for styling the native dom elements. The classes.root need to be applied as a className. The current API ask for the same "hack" than with styled-components: https://material-ui.com/guides/interoperability/#styled-components. I'm not convinced documenting your version would be a better default, while it's definitely interesting.

[mssngr]

Good feedback, thanks! And, I don't know how I missed that page of the documentation. I've gotta rethink my styling strategy, now.