WIP: Add dynamic helper instance methods

I was trying to make a translation page (two languages side by side in a form) and I realized it was not possible to give `title_i18n["fr"]` to the form helper. So I set out on this great quest to make some dynamic helpers so I could use `title_fr` when `I18n.locale == :en`, alas it's not that simple.

So here it is, in all its WIP glory, so you can give your comments and maybe make this a real feature.