Feat. Postprocessing control - custom page separator, postprocess function etc

[pradhyumna85]

To accommodate and resolve #37

Changes

Note: This PR adds changes on top of PR #39. If merged, this will accommodate changes of PR #39, which won't require the previous PR to be merged.

• added post_process_function param to override/skip Zerox's default format_markdown post processing on the model's text output.
• removed output_dir param and added output_file_path which is more flexible for arbitrary file extensions
• page_separator param added (used when writing the consolidated output to the output_file_path
• +Added way to skip validation of model's vision capability as model might not be added to the litellm static json config which has a list of models and their capabilities. https://github.com/BerriAI/litellm/blob/main/model_prices_and_context_window.json

Edit: ~~Fixes #42~~