Create a code example for adding the subscription widget to Subscribe Pro enabled products in a Grouped product.

[chrismshea]

PR: #270

From time to time we have customers request a code example of adding the subscription widget to to the products that are being displayed in a group. It would be nice to have one on file.

[garthbrantley]

@eugene-shab Please add this feature to each simple product item on the grouped product page.

So for example this page would get 3 subscription widgets:

[eugene-shab]

@garthbrantley
Added functionality for displaying subscriptions on the Grouped Product pages.

To display subscriptions, you need to create a grouped product and add simple products that have a subscription enabled.

Simple products that have a subscription disabled will be displayed as regular products