Adding option for hooks after postprocessors to YoutubeDL

[alexmerkel]

Before submitting a pull request make sure you have:

• Searched the bugtracker for similar pull requests
• Read adding new extractor tutorial
• Read youtube-dl coding conventions and adjusted the code to meet them
• Covered the code with tests (note that PRs without tests will be REJECTED)
• Checked the code with flake8

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• I am the original author of this code and I am willing to release it under Unlicense
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What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

This pull request adds an option called post_hooks to youtube_dl/YoutubeDL.py. The option takes a list of callable functions. The functions will be called as almost the last step in the download process, after all postprocessors are done. They have to accept the filename as an argument.
Additionally, a test for the post hooks functionality is added in this pull requests. As the test contains downloads, it is added to the respective lists in the Makefile and the run_tests scripts.