Skip attaching handler to passed (options.skip) tab indices

Skips attaching event handler to specified tabs.

Uses include: external links, custom actions such as modals, etc.

Example usage:

````new CBPFWTabs( document.getElementById( 'tabs' ), {'skip' : [2, 3]} );````

**NB**: Array.indexOf is compatible with IE9 and above see [MDN documentation](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/indexOf).