Store maximum number of paralle instances as addon config
Store maximum number of paralle instances as addon config

This fixes #5.
As number of background parallel workers is most likely going to be
computer specific rather than scene (or even project) specific I've made
it an addon preference that is not save per scene.
You can now set default value in User Perferences -> Addons -> Parallel
Render -> expand it and set the value then save user perferences.

elmopl/ktba,elmopl/ktba