Custom Tasks should be possible

[ghost]

Add the ability to register custom tasks. This is entirely possible with celery, but there's not hookup mechanism yet. Custom task classes should be specified in a ENV var tuple and added to the chain. It should be possible to specify when the tasks run (pre_convert, post_convert, etc. maybe using a signal system for hookup).

[ghost]

Most likely this will happen with a reworked template bundle system. So the entire process can be customized.