Add post hook

[hugopeek]

This allows you to run a custom snippet after the thumbnail has been generated. Useful for further optimization with other libraries.

This PR needs to implement the pthumb.post_hook system setting still, but I don't know how to do that in the current configuration. It's very simple with GPM, so I vote to merge https://github.com/modxcms/pThumb/pull/68 from @joeke first.