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

[TheSpaceSheep]

This PR fixes #2160.
Filenames that starts with dash '-' are no longer interpreted as command line arguments by ffmpeg

• I have provided code that clearly demonstrates the bug and that only works correctly when applying this fix
  See ffmpeg interpets video title as a command line argument when it begins with "-" #2160
• 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

[coveralls]

coverage: 81.702% (-0.2%) from 81.904%
when pulling 7216d6e on TheSpaceSheep:fix_2160
into bc8d1a8 on Zulko:master.