Discussion on how to integrate with Babel's ability to use recast

So after https://github.com/babel/babel/pull/3561 allows babel to use recast as the parser/generator and the other prs fixed the babel 6 ast nodes for ast-types/recast we should be able to use babel as the transformer for jscodeshift.

Is there anything we want to do to the api to make it easier or should users just add it in themselves? babel-core/recast are already dependencies, but yeah it's not necessary (*Ah* I see babel-core is v5 for the babel 5 parser). Maybe it's just a docs change.

```js
module.exports = function(fileInfo, api) {
return api.jscodeshift(fileInfo.source)
.findVariableDeclarators('foo')
.renameTo('bar')
.toSource();
}
```

```js
const transform = require('babel-core').transform;
const recast = require('recast');
const plugin = require('./plugin');

module.exports = function(fileInfo, api, options) {
return transform(fileInfo.source, {
parserOpts: {
parser: recast.parse
},
generatorOpts: {
generator: recast.print
},
plugins: [plugin]
}).code;
};

// ./plugin.js
module.exports = {
visitor: {
VariableDeclarator({ node }) {
if (node.id.name === "foo") {
node.id.name = "bar";
}
}
}
};
```

Also interesting sidenote would be using the jscodeshift type api in babel

cc @fkling, @cpojer, @drewml