Add `_update_settings.json` feature to allow proper index update

manage index creation settings in `_settings.json` file and updatable settings in `_update_settings.json` separate file:

In your case, it means for `_settings.json` file:

```javascript
{
  "index" : {
    "number_of_shards" : 3,
    "number_of_replicas" : 0
  }
}
```

And in `_update_settings.json`:

```javascript
{
  "index" : {
    "number_of_replicas" : 1
  }
}
```

So, when the index does not exist, I create it using `_settings.json` file. If `mergeSettings` is `true`, then I apply each time the factory starts `_update_settings.json` file.

Closes #31.