Add hook for post processing

[tcely]

We have one for downloading already, with SponsorBlock working it can take a while.

It's better to log something about how that is going.

postprocessor_hooks:  A list of functions that get called on postprocessing
                       progress, with a dictionary with the entries
                       * status: One of "started", "processing", or "finished".
                                 Check this first and ignore unknown values.
                       * postprocessor: Name of the postprocessor
                       * info_dict: The extracted info_dict

                       Progress hooks are guaranteed to be called at least twice
                       (with status "started" and "finished") if the processing is successful.