Added apply_filters() to edd_show_added_to_cart_messages() function

First github contribution ever!

I found myself wanting to alter the message that appears after adding a download to the cart. I couldn't find any apply_filters() filter hook to use so decided to add one. 

This will allow a developer to alter the underlying HTML and the message itself. In my case I want to be able to add a CSS class name and alter the default text completely.