avformat/dashenc: Added option for Segment file format

Right now segment file format is chosen to be either mp4 or webm based on the codec format.
This patch makes that choice configurable by the user, instead of being decided by the muxer.

Also with this change per-stream choice segment file format(based on codec type) is not possible.
All the output audio and video streams should be in the same file format.

doc/muxers.texi

@@ -288,6 +288,15 @@ the segment indexes fall behind the expected real time position.
 Set container format (mp4/webm) options using a @code{:} separated list of
 key=value parameters. Values containing @code{:} special characters must be
 escaped.
+
+@item dash_segment_type @var{dash_segment_type}
+Possible values:
+@item mp4
+If this flag is set, the dash segment files will be in in ISOBMFF format. This is the default format.
+
+@item webm
+If this flag is set, the dash segment files will be in in WebM format.
+
 @end table
 
 @anchor{framecrc}