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;
```

1. サポートしている拡張子の配列 `supportedExts` を定義する
2. `supportedExts` をループして、実際に存在するファイルパスを返す `getReadablePath` 関数を定義する

という方法はいかがでしょうか。

```js
var supportedExts = [".js", ".json", ""];
function getReadablePath(filePath, exts) {
    for (var i=0; i < exts.length: i++) {
        var pathWithExt = filePath + exts[i];
        if (fs.existsSync(pathWithExt)) {
            return pathWithExt;
        }
    }
    throw new Error (filePath + "not found.");
}
function inliningModule(filePath, entryData, opt) {
    // ...（略）...
    var readablePath = getReadablePath(filePath, supportedExts);
    var src = fs.readFileSync(readablePath, "utf-8");
```