Support video format for tensorflow-io

[yongtang]

In TensorFlow, video format is supported through tf.contrib.ffmpeg which calls command line ffmpeg to decode video format to tensors and feed into tensorflow.

The tf.contrib.ffmpeg is pretty much unmaintained, and, the command line ffmpeg invocation is really unreliable due to the changes of output text over different versions.

The tf.contrib.ffmpeg will also be deprecated soon so users of tensorflow will have no direct access to video format very soon. This is a big loss for many users.

I think it makes sense to support video formats in tensorflow-io, by dynamically linking ffmpeg's library (not command line invocation) and generate output to tf.data.

We have to be very careful with licenses for external libraries, though as far as I know (correct me if I am wrong), ffmpeg is LGPL 2.1+ so it would be OK to only dynamically linking ffmpeg from tensorflow-io (Apache 2.0 license). Also, we should not distribute ffmpeg library directly. We should merely call the api through .so/.dll if it has been installed in system already.

[yongtang]

Added a PR #30 to support video files. The PR is pretty much working though requires Ubuntu 18.04 (exact version). Some additional work will needs to be done to make it work in more platforms.