Zombie processes after reader.close()
Hi,

When MoviePy finishes rendering I'm still left with a lot of zombie ffmpeg processes. (http://cl.ly/image/1X2h0o0J103K)

These don't use any resources, but there is a maximum nr of processes ofcourse. 
Is this bad? Can this be overcome?

diff --git a/moviepy/video/io/ffmpeg_reader.py b/moviepy/video/io/ffmpeg_reader.py
index 70b098d..420eed1 100644
--- a/moviepy/video/io/ffmpeg_reader.py
+++ b/moviepy/video/io/ffmpeg_reader.py
@@ -169,6 +169,7 @@ class FFMPEG_VideoReader:
             self.proc.terminate()
             self.proc.stdout.close()
             self.proc.stderr.close()
+            self.proc.wait()
             del self.proc
 
     def __del__(self):
['moviepy/video/io/ffmpeg_reader.py']