Support invalid format tags in subrip decoder

[Carl Eugen Hoyos]

​http://thread.gmane.org/gmane.comp.video.ffmpeg.user/58476
A user provided a subrip file with ass (?) format tags, they are currently ignored by FFmpeg.

$ ffmpeg -i tlf-trumanshow.bd.Eng.3d.srt
ffmpeg version N-74676-gf5258a7 Copyright (c) 2000-2015 the FFmpeg developers
  built with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl
  libavutil      54. 31.100 / 54. 31.100
  libavcodec     56. 58.100 / 56. 58.100
  libavformat    56. 40.101 / 56. 40.101
  libavdevice    56.  4.100 / 56.  4.100
  libavfilter     5. 38.100 /  5. 38.100
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  2.101 /  1.  2.101
  libpostproc    53.  3.100 / 53.  3.100
Input #0, srt, from 'tlf-trumanshow.bd.Eng.3d.srt':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Subtitle: subrip
At least one output file must be specified