Add forceArray config option

Add `forceArray` config option that, when set to `true`, will wrap the node in an array if it is a map.  Useful when transforming JSON that was converted from XML, in which case a node that may normally contain multiple items will be converted to a JSON map if it only contains a single item.

For example, an XML to JSON converter will typically convert the XML:

```xml
<breakfast_menu>
  <food>
    <name>Belgian Waffles</name>
    <price>$5.95</price>
  </food>
</breakfast_menu>
```

to the JSON:

```json
{
  "breakfast_menu": {
    "food": {
      "name": "Belgian Waffles",
      "price": "$5.95"
    }
  }
}
```

The `forceArray` config option can be used to rectify this:

```js
new json2json.ObjectTemplate({
  path: 'breakfast_menu.food',
  all: true,
  forceArray: true
}).transform(json)
```

```[ { name: 'Belgian Waffles', price: '$5.95' } ]```