Updated support for annotating video frames

Using the actively maintained project moviepy (https://github.com/Zulko/moviepy), I have added support for annotating video frames, with a simple example of how to write the JSON specification for including frames. 
I also included moviepy as a pip-installable dependency, so users should automatically have it installed, and not need to fish for dependencies. A caveat is it requires an installed version of ffmpeg, but all modern Linux systems include this, and it is easily installable on OSX. If the maintainers of sloth felt it was required, I could write a small bit of documentation highlighting how to troubleshoot the installation of ffmpeg. 

I also did not use the `_video_cache` object at all in my code, instead opting for a seek-and-read strategy for single frames. It performs decently well on the videos (all tens of minutes long) that I have tried. However, caching might be desirable in the future. 

Please let me know any comments or tips for improvement. Thanks for the great tool, would be excellent to contribute to it.

You should also advertise! It took me forever to find out this tool exists, and it's very useful for my community.