add afterimage example

tests/test_Videos.py

@@ -0,0 +1,25 @@
+import pytest
+from moviepy.editor import *
+
+# must have to work on travis-ci
+import sys
+sys.path.append("tests")
+import download_media
+
+def test_download_media(capsys):
+    with capsys.disabled():
+       download_media.download()
+
+def test_afterimage():
+    ai=ImageClip("media/afterimage.png")
+    masked_clip = vfx.mask_color(ai, color=[0,255,1]) # for green
+
+    some_background_clip = ColorClip((800,600), color=(255,255,255))
+
+    final_clip = CompositeVideoClip([some_background_clip, masked_clip],
+                                    use_bgclip=True)
+    final_clip.duration=5
+    final_clip.write_videofile("/tmp/afterimage.mp4", fps=30)
+
+if __name__ == '__main__':
+   pytest.main()