v1.5.1

Added

• Added a runtime flag to use decoding timestamp in timeline for mp4:
  --mp4_use_decoding_timestamp_in_timeline

Changed

• Remove restriction that sps:gaps_in_frame_num_value_allowed_flag should be
  0 in h264. Packager should not care about this flag (#126).
• Remove restriction that sample duration cannot be zero. A warning message
  is printed instead (#127).

Fixed

• Fix text formats (webvtt, ttml) not recognized problem (#130).