fix tab deep-linking

This should fix the popular issue https://github.com/zurb/foundation/issues/4791.

I used an approach using prefixes for the hashes so that the browser doesn't autoscroll.
This removes the necessity of remembering the scroll position and scrolling back there on tab change, which doesn't work great across browsers. Despite using prefixes **this PR does not change the API in any way**, except the way the hash is shown in the address bar.

The name of the prefix (currently 'fndtn') is open for discussion :)

If you think you found some errors in my code or can suggest any improvements, please feel free to file a pull request in my fork as well. Maybe I'm going to add tests for this in the near future (seems like tabs aren't tested at all at the moment, so there's room for that).