Add email capture component

[ndarilek]

I'd like a component similar to the signup component, but for just requesting an email address for subscribing to a MailChimp or other list.

I guess this is somewhat related to #8, but in the absence of a solution there, capturing emails is one feature I was surprised to see missing.

Thanks for this, looks like a very neat project!

[dennybritz]

Good idea. I'll see if I can get it done over the holidays!

[dennybritz]

Added this in 10cccc5. You should now be able to write something like:

<SignupModal modalId="signup-modal" onSubmit={(data) => console.log(data)}>
  <SignupModal.Input name="email" type="email" required label="Email" placeholder="Email Address"/>
</SignupModal>

This will render the modal with only an email field.