WebVTT subtitle tracks have four kinds. Certain downstream muxers
(such as for WebM) need to know which WebVTT kind this is, in order to
specify the codec id of the output track.

A new private input option, "-kind", has been added to the WebVTT
demuxer.  It accepts as a value any of "subtitles" (the default),
"captions", "descriptions", and "metadata".  The kind option value is
used to assign a value to the stream disposition flag, to which four
new values have been added, corresponding the four WebVTT kinds.

Signed-off-by: Matthew Heaney <matthewjheaney@google.com>
Signed-off-by: Clément Bœsch <ubitux@gmail.com>