CLI: Add --esprima option. #708

Currently you cannot use `csjs` with [JSX](https://github.com/facebook/jsx) (ReactJS) files:

```js
var React = require('react');
module.exports = React.createClass({
  render() {
    return <div>Hello</div>;
  }
});
```

```
Unexpected token < at ./src/components/test.jsx :
      3 |  render() {
      4 |    return <div>Hello</div>;
--------------------^
```

If you could provide a custom [Esprima](http://esprima.org/) version via CLI or `.jscsrc` file, that would fix the problem:

at the CLI:

`jscs --esprima=esprima-fb`

in `.jscsrc`
```json
{
  "esprima": "esprima-fb"
}
```
Right, I guess `config` is always defined at this point.