Add requiredFirst option to jsx-sort-prop-types

[chrislaskey]

Hey there, I've been really enjoying eslint-plugin-react, thanks for creating such a great set of React rules for ESLint!

Our team likes to put required prop type declarations first, followed by optional prop types. I extended the existing jsx-sort-prop-types rule to take a requiredFirst option to make this enforceable by ESLint.

When set to true required props must come first and be sorted alphabetically:

class First extends React.Component {
  render() {
    return <div />;
  }
}

First.propTypes = {
  barRequired: React.PropTypes.string.isRequired,
  fooRequired: React.PropTypes.any.isRequired,
  a: React.PropTypes.any,
  z: React.PropTypes.string,
  onBar: React.PropTypes.func,
  onFoo: React.PropTypes.func
};

In addition to the changes this PR includes tests and documentation.

Figured others could find it useful too. 😃

[yannickcr]

Great! Thanks!