Add stream selection support for `tf.contrib.ffmpeg.decode_audio`

This fix tries to address the issue raised in 16073 where it
was not possible to selectively decode a perticular stream
with `tf.contrib.ffmpeg.decode_audio`.
This fix adds an additional attribute `stream` which could
be used to specify the stream to decode. By default stream=''
which leaves the decision to ffmpeg.

This fix fixes 16073.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>