video conversions fail with space in filename

[gangelop]

this

sections:
    images:
      -
         - type: video
           name: "cool video.mp4"

causes this

Generation: ./gallery1/cool video.mp4
ffmpeg -loglevel error -i ./gallery1/cool video.mp4 -c:v libvpx -b:v 3900k -qmin 10 -qmax 42 -maxrate 500k -bufsize 1500k -c:a libvorbis -b:v 100k -s 1280x720 -f webm -y "./build/gallery1/cool video.mp4.webm"
./gallery1/cool: No such file or directory
Abort: ffmpeg command failed

Some quotes are needed for {source} and {target} in prosopopee/prosopopee.py.