Vue-storefront: Add support for virtual products

[pkarw]

I mean very simple support: if you've got only virtual products in the cart, no shipping address should be required for the checkout. Is meant for orders of digital media/gift cards etc

[kyvaith]

This should be provided with extensible "custom action" after purchase. In example the purchased ebook should open in a web player.

[ananth-iyer]

@pkarw I am working on this issue

[ananth-iyer]

@pkarw @filrak Please review my changes and let me know if you have questions: f94348e

[patzick]

Thank you @ananth-iyer :)

[ananth-iyer]

Hurray! I got my first PR merged to develop

[patzick]

Yeah, that feeling is great! ;) Keep it up, it get's even better!