Add a contextual component for sharing elements between stripe-card-* elements

[lindyhopchris]

As shown in #14, to get the individual stripe-card-* elements working, you actually need them to be backed by the same stripe.elements() instance.

This is pretty confusing, as shown by some of the questions in #9

The way to solve it is to have a contextual component that does that wiring for you, example use:

{{#stripe-elements as |elements|}}
   {{elements.cardNumber}}
   {{elements.cardExpiry}}
{{/stripe-elements}}

Need to flesh it out, but something along those lines. Would also mean options could be passed using {{#stripe-elements options=options}} which would solve #33

I'll put together a PR when I'm next working on Stripe related stuff, probably this weekend. But posting here so if anyone has any thoughts/requirements they can comment below.