Make cv2-based processes autoconvert to avi if needed

[balintlaczko]

At the moment these are:

• average
• flow
• motion
• show

[balintlaczko]

Update: I will exclude show from this list, since I decided to re-implement it with the windowed mode using ffplay and the inline (notebook- and colab-friendly) mode using Ipython.display.Image (issue #121) and an HTML video tag using Ipython.display.HTML (issue #135).