Add Post Processors & Example PDF Redaction

[JSv4]

Upgrades to pipelines:

1. Added a post-processor type that's meant to be used on the export step on the assembled zip. Error handling is simplistic so this will currently choke on really large corpuses.
2. Add an example post processor that redacts pdfs based on selected annotations.
3. Add an input_schema class property to pipeline classes so we can clearly define class-specific inputs and potentially collect this via GUI.
4. Implementing input_schema functionality just for post-processor (and pdf redactor specifically) atm so you can select which annotations to treat as redactions.
5. Changing to a Copyleft license for now... particularly as I'm starting to add functions that are paid features in other tools/products. I want this to be a community asset, but I also don't want to be free R&D. Could be convinced to return to Apache2 with enough community engagement.