This commit restores the ability to create DASH streams with codecs
that require different containers that was lost after commit
2efdbf73. It adds a new "auto" value for
the dash_segment_type option and makes it the default. When in this mode,
the segment format will be chosen based on the codec used in the stream:
webm for Vorbis, Opus, VP8 or VP9, mp4 otherwise.