By default FFmpeg assumes a 25 FPS input frame rate, leading to dropped
frames if the source requires a higher frame rate than that.
This commit adds a `framerate` option (default "auto"), which allows to
automatically assign a (more or less) fitting frame rate based on
delays between ugoira frames and avoids dropped frames.
- ffmpeg-location: path to the ffmpeg (or avconv) executable
- ffmpeg-args: additional command line args for ffmpeg
- extension: filename extension of the resulting video file
