Add data-random support to carousel

This new data attribute lets you specify that a carousel should start with a random active item instead of the one marked active in the page DOM.  With this, you don't even need to mark any item as active which actually improves the look, since nothing will show before the first random item is picked.

On my own site, I used this for a carousel of site sponsor logos, with the randomness meaning no sponsor always gets first billing.