allow payloads to be propagated to new tasks

This change set adds the ability to propagate payloads from received tasks to new tasks for extracted files.

This allows e.g. to specify in the parent task that the file came originally from an e-mail.

task.add_payload('ext_origin_id', 'email', persistent=True)

Any downstream karton consumers can use these propagated payloads to fine-tune their decision making. A simple example is provided below. We want to flag .docm files in a .zip archive received over e-mail:

sample = task.get_resource('sample')
ext_origin_id = task.get_payload('ext_origin_id')
extraction_level = task.get_payload('extraction_level')
if '.docm' in sample.name and extraction_level and ext_origin_id == 'email':
    print('.docm files extracted from .zip over email are considered super suspicious')

CERT-Polska / karton-archive-extractor

allow payloads to be propagated to new tasks #56