Reintroduce customer shipping forms

I understood the choice has been made to drop shipping forms support. So I wrote my own ShippingMethod class, and it works just fine. But doing so I discovered it takes 4 easily maintained lines of code and an almost empty interface to support the optional addition of shipping forms. I made the pull request because it saves me the hassle of keeping an extension up to date because I'd need to check changes in the generate function on every Isotope update and because I think some important functionality is lost!

The option of showing an extra input form for any shipping method is massively purposeful. Consider a gift-store built using Isotope 1. It features three shipping methods: Kiala, Home delivery by date and Download/Email. Each of these options has a unique set of fields the customer needs to fill in, Kiala and Date must be done before placing an order. For example; delivery by date can definitely not be shown after placing the order; customers might want a delivery date that we cannot guarantee, but download/e-mail never needs a date, that is just pointless. Instead it needs an email address, sender address and gift message. Kiala needs customers to pick where we send the package to, but the customer may not be aware beforehand if they can get to any of them.

Can you please reconsider?