Extend autopkg capabilities by adding a before and after hook system

Extend autopkg capabilities by adding a before and after hook system allowing system administrator to add custom processor to existing recipes from the command line, without having to override them.

Tested for some weeks now on my setup, seems to work well.

Before processor are added via -b and after via -b. Multiple processor of each kind can be added.

If the pre and post processor need inputs, they can use the -k option or use any existing file on disk.

The MunkiSetDefaultCatalog processor added with this commit is an example of that. It read the default catalog set in the munki settings file for the current user and update the recipe input settings to override the target catalog before the recipe execution and without needs of new recipe overwrite file on disk.