Highlight first TOC entry by default

Having the option to have the first TOC entry be marked as `active`, by default, would be a great addition.

Ideally, this would occur when a fresh webpage is first loaded, without an anchor tag location.

This issue can also be seen via the example in:
https://github.com/joaquincasares/jquery.tocify.js/tree/Issue-20/Issue-20
