Added 'data' attribute

[wmerfalen]

Hello.

I recently used your plugin in a project. I wanted a bit more flexibility when adding content to the popover. I implemented a simple hack to allow the user to create their own content in the popover.

Example:

        jQuery("#popoverDiv").optionsPopover({
            id: 'popoverDiv',
            title: 'Hello world!',
            contents: [
                {'name': 'Google', 'url': 'https://www.google.com/'},
                {'data': '<h3>Custom content :)</h3>'}
            ]
        });