JSX Highlighting for the Highlighting Gods

JSX and TypeScript are both getting pretty darn popular, we should definitely support highlighting their syntax in READMEs!

use ```jsx for JSX highlightning:

class Thinger extends React.Component {
  constructor(props) {
    super(props)
  }
  render() {
    return (
      <div>{this.whatever}</div>
    )
  }
}

use ```ts or ```typescript for TypeScript highlighting:

declare namespace myLib {
    function makeGreeting(s: string): string;
    let numberOfGreetings: number;
}

declare namespace myLib {
    function makeGreeting(s: string): string;
    let numberOfGreetings: number;
}