Tab navigraion class / element as option

Currently, all ````nav```` descendants of the passed element are also bound to the click event on init. This may include multiple ````nav````s inside of tabs used for navigating a ````section```` or filtering content.

Proposed example usage:

````new CBPFWTabs( document.getElementById( 'tabs' ), {'nav' : '.nav-main'} );````

The default if no class or element is passed is ````nav```` as before, which grabs all the nav descendants, though it may perhaps be better to use a specific class (such as ````.nav-main```` in the example above) but this would require updating the HTML of the original article.