Add hooks before and after the subscription info table email

[webdados]

Fixes #752

Description

As mentioned on the issue:

Depending on the subscription business, the information on the "Subscription information" email template might not be enough for the customer to understand why is he getting this email.

(...)

I have two suggestions:

• Load the order details email template after the subscription information table
• Add hooks before and after the subscription information table so that developers can decide what else to add

This PR implements the second suggestion.

How to test this PR

Hook into wcs_email_before_subscription_info or wcs_email_after_subscription_info and add any content you want to the subscription information template on emails.

Product impact

• Will this PR affect WooCommerce Subscriptions?