option to use a custom ffmpeg path

[frueter]

It would be convenient if scenedetect could be pointed to a custom ffmpeg path to use for media export.
Currently I add it to PATH which is fine, but was just thinking it coud be nice to include this scenario in the API.
os.environ["PATH"] = str(ffmpeg_parent_dir) + os.pathsep + os.getenv("PATH", "")