Detect audio posters in probe

This is related to #25 , when `probe` incorrectly report video stream for audio files with poster images. (Note that mp3 files with poster image can be converted into a complete video!)

Now, `probe` functions has a separate argument `posters_as_video` to change this behavior.
With `True` (default value), the behavior is as has been previously, the poster image is accounted as a video stream, and so `info.video` may return poster images as well as video steams.

But if we pass `False` as `posters_as_video`, `info.video` will be None, or a audio file with poster(s).

And finally, all poster images all always accessible via `info.posters`