(fix) escape dashes in the beginning of filenames in ffmpeg commands

[TheSpaceSheep]

This PR fixes https://github.com/Zulko/moviepy/issues/2160. Filenames that starts with dash '-' are no longer interpreted as command line arguments by ffmpeg

• [x] I have provided code that clearly demonstrates the bug and that only works correctly when applying this fix See https://github.com/Zulko/moviepy/issues/2160
• [x] I have added suitable tests demonstrating a fixed bug or new/changed feature to the test suite in tests/
• [x] I have properly documented new or changed features in the documentation or in the docstrings

[coveralls]

coverage: 81.702% (-0.2%) from 81.904% when pulling 7216d6ea9695e247dd9b4255726e7cc7347bd30f on TheSpaceSheep:fix_2160 into bc8d1a831d2d1f61abfdf1779e8df95d523947a5 on Zulko:master.