Make it possible to pass options to cart_form helper

It allows to customize form behavior (i.e. async processing, styling) without overriding the whole view

I needed such a feature while trying to make the add to cart button not to load the cart page but to let the customer continue shopping, while asynchronously processing the cart. A `data-remote` attribute was the easiest way to handle it IMO.

I think that it can be useful in most cases.
Let me know !

Cheers