Quote filenames to handle paths with spaces in commands created with `.compile()`

Problem

ffmpeg-python's .compile() method to generate FFmpeg commands doesn't quote or escape filenames with spaces, so it will generate commands like so:

>>> ' '.join(output.compile())
ffmpeg -fflags +genpts -i /tmp/video with spaces in name.mkv \
 -acodec copy -movflags faststart -scodec copy -threads 12 -vcodec libx264 -vlevel 4.1 \
 /tmp/video with spaces in name [out].mkv -y

(Note that I edited escaped new lines into this example's output for readability, they don't exist in .compile()'s output before or after merging this pull request.)

Running that compiled command fails:

/tmp/video: No such file or directory

Solution

This pull request quotes filenames using shlex.quote() by wrapping filenames parsed by .compile()'s helper functions.

As a result, the following command can run correctly because filenames with spaces were quoted:

>>> ' '.join(output.compile())
ffmpeg -fflags +genpts -i '/tmp/video with spaces in name.mkv' \
  -acodec copy -movflags faststart -scodec copy -threads 12 -vcodec libx264 -vlevel 4.1 \
  '/tmp/video with spaces in name [out].mkv' -y

(Again, escaped new lines were just added to this example for readability, this pull request does not add them to output.)