Added support for partials object

jQuery Mustache plugin was missing support for partial objects which are supported by Mustache. From Mustache docs:

> In mustache.js an object of partials may be passed as the third argument to `Mustache.render`. The object should be keyed by the name of the partial, and its value should be the partial text.

> ```js
> Mustache.render(template, view, {
>   user: userTemplate
> });
>```

The patch adds support for such object. This allows to choose partial templates dynamically. For example, if template has partial with name `{{> content}}`, it is now possible to render some differently named template in place of this partial.

Here's an usage example:

```html
        <script type="text/html" id="div">
            <div>
            Div content:
            {{> content}}
            </div>
        </script>
        <script type="text/html" id="content1">
            Hello world!
        </script>
        <script type="text/html" id="content2">
            Bye world!
        </script>
```
```js
    $('body').mustache('div', {}, {}, {content: 'content1'});
    $('body').mustache('div', {}, {}, {content: 'content2'});
```
And the result:
```
Div content: Hello world!
Div content: Bye world!
```