Allow altering classes before render.

Problem/Motivation

During the work, I had the need to dynamically change the classes depending on the settings of the node.
I couldn't find the hook that would work the way I needed it to.
So the idea came up to add a hook call: 'hook_ds_classes_alter' before render.
I did make a small patch that implements this.

Not sure how correct this is a solution.
However, it may be useful.