Add filter to ajax discount code

This allows for plugins to create discount "aliases", thus preventing the need to create tons of different discount posts.

Example use case:

A user enters their existing license key. A plugin could validate the license key and, if successful, replace it with the (existing) "RENEW" discount code.
Filter the $return array after it's been processed
Update ajax-functions.php
Renamed filter to `edd_ajax_discount_response`
Merge pull request #2119 from mgibbs189/master

Add filter to ajax discount code