Added ffmpeg download when importing moviepy.editor (#731)

When the `FFMPEG_BINARY` environment variable is not set, the ffmpeg binary will be downloaded if needed when `import moviepy.editor` is called.

moviepy/editor.py

@@ -17,8 +17,15 @@
 # Note that these imports could have been performed in the __init__.py
 # file, but this would make the loading of moviepy slower.
 
-# Clips
+import os
+
+# Downloads ffmpeg if it isn't already installed
+import imageio
+# Checks to see if the user has set a place for their own version of ffmpeg
+if os.getenv('FFMPEG_BINARY', 'ffmpeg-imageio') == 'ffmpeg-imageio':
+    imageio.plugins.ffmpeg.download()
 
+# Clips
 from .video.io.VideoFileClip import VideoFileClip
 from .video.io.ImageSequenceClip import ImageSequenceClip
 from .video.io.downloader import download_webfile