Add 'loop' argument support writing GIFs with ffmpeg

[mondeja]

• Add tests for moviepy.video.io.gif_writers::write_gif and document it.
• Add documentation for moviepy.video.io.gif_writers::write_gif_with_tempfiles.
• Allow to pass loop argument for both functions when program="ffmpeg" using loop FX.
• Refactor tests removing test_Videos.py file (test moved to test_VideoClip.py).

• I have provided code that clearly demonstrates the bug and that only works correctly when applying this fix
• I have added suitable tests demonstrating a fixed bug or new/changed feature to the test suite in tests/
• I have properly documented new or changed features in the documentation or in the docstrings
• I have properly explained unusual or unexpected code in the comments around it

[coveralls]

Coverage increased (+0.06%) to 83.04% when pulling 6f207de on mondeja:write_gif-tests into 2aff1cf on Zulko:master.