Add support for requiring JSON files

`require` can read pure JSON files, in addition to JavaScript files.
However, currently *inlining-node-require* cannot concatenate pure JSON files into JavaScript files properly.
So I updated `lib/inlining.js` to support JSON requiring and added a unit test for this feature.

#### Example

##### Sources

```json
{"hi": "This is pure JSON."}
```

```js
var data = require('path/to/json');
var greeting = data.hi;
```

##### Result

```js
var data = {"hi": "This is pure JSON."};
var greeting = data.hi;
```