Adding an option to load method for specifying array nodes

### Issue

Currently, load() method creates array object for a node only if there are more than 1 sibling nodes with the same tag name.

In the sample below, the data structure for both <items1> and <items2> are supposed to be the same - arrays. And we know that beforehand, just there is no way for us to tell load() method that we want items1.item and items2.item to always be array, no matter of the <item> tag count.

Input:
```xml
<items1>
    <item>1</item>
</items1>
<items2>
    <item>1</item>
    <item>2</item>
</items2>
```
Output#1
```javascript
{
  "items1": {
    "item": {
      "$t": "1"
    }
  },
  "items2": {
    "item": [
      {
        "$t": "1"
      },
      {
        "$t": "2"
      }
    ]
  }
}
```

### Solution

In this PR, I added a new option in options object - "arrays". For the same xml input above, and with the options below:

```javascript
var xml = xmlSpecifiedAbove;
var options = {
  arrays: [
    '/items1/item',
    '/items2/item'
  ]
};
xm.load(xml, options);
```
Will return expected json:

```javascript
{
    "items1": {
        "item": [{
            "$t": "1"
        }]
    },
    "items2": {
        "item": [{
            "$t": "1"
        }, {
            "$t": "2"
        }]
    }
}
```