Add a quick buy button

Add an option for 'quick buy button' to allow user in one click:

• add a product to shopping cart;
• redirects user to /checkout page.

A client of mine required this feature and I created a simple sandbox project (https://www.drupal.org/node/2012412). But it's too simple and maybe it's not a good idea to have it as a single module. Maybe it's better to include it as a feature to Commerce?

I can create a patch with this feature included.