Support overriding fps in --ffmpeg-config flag

fps value still defaults to source fps, but you can now override it
to e.g. 15 through --ffmpeg-config='{"fps": 15}'.

Thanks for the report @Karl48071

Closing ORB-HD#24

deface/deface.py

@@ -136,8 +136,12 @@ def video_detect(
         bar = tqdm.tqdm(dynamic_ncols=True, total=nframes)
 
     if opath is not None:
+        _ffmpeg_config = ffmpeg_config.copy()
+        if not 'fps' in _ffmpeg_config:
+            #  If fps is not explicitly set in ffmpeg_config, use source video fps value
+            _ffmpeg_config['fps'] = meta['fps']
         writer: imageio.plugins.ffmpeg.FfmpegFormat.Writer = imageio.get_writer(
-            opath, format='FFMPEG', mode='I', fps=meta['fps'], **ffmpeg_config
+            opath, format='FFMPEG', mode='I', **_ffmpeg_config
         )
 
     for frame in read_iter: