Add GST_TAG_TRACK_KIND = captions to cea708 captions.

This depends on [pull request #3 in gst-plugins-base](https://github.com/cablelabs/gst-plugins-base/pull/3).

I tried adding this in gstccdec.c, but the tag didn't show up. I'll come up with a cleaner way to handle this when the GStreamer developers make up their mind about this tag.