default to opus codec for *.opus output files

[LinAGKar]

Summary of the bug:
Presumably since .opus files uses he ogg container they will default to flac audio, however .opus files are expected to contain opus streams.

How to reproduce:

% ffmpeg -i input output.opus

example:

> ffmpeg -i C418\ -\ Minecraft\ -\ Volume\ Alpha\ -\ 01\ Key.flac -vn C418\ -\ Minecraft\ -\ Volume\ Alpha\ -\ 01\ Key.opus
ffmpeg version 2.1.1 Copyright (c) 2000-2013 the FFmpeg developers
  built on Nov 20 2013 20:26:43 with gcc 4.7 (SUSE Linux)
  configuration: --shlibdir=/usr/lib64 --prefix=/usr --mandir=/usr/share/man --libdir=/usr/lib64 --enable-shared --disable-static --enable-debug --disable-stripping --extra-cflags='-fmessage-length=0 -O2 -Wall -D_FORTIFY_SOURCE=2 -fstack-protector -funwind-tables -fasynchronous-unwind-tables -g -fPIC -I/usr/include/gsm' --enable-gpl --enable-x11grab --enable-version3 --enable-pthreads --enable-avfilter --enable-libpulse --enable-libvpx --enable-libopus --enable-libass --enable-libmp3lame --enable-libvorbis --enable-libtheora --enable-libspeex --enable-libxvid --enable-libx264 --enable-libschroedinger --enable-libgsm --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-postproc --enable-libdc1394 --enable-librtmp --enable-libfreetype --enable-avresample --enable-libtwolame --enable-libvo-aacenc --enable-gnutls
  libavutil      52. 48.101 / 52. 48.101
  libavcodec     55. 39.101 / 55. 39.101
  libavformat    55. 19.104 / 55. 19.104
  libavdevice    55.  5.100 / 55.  5.100
  libavfilter     3. 90.100 /  3. 90.100
  libavresample   1.  1.  0 /  1.  1.  0
  libswscale      2.  5.101 /  2.  5.101
  libswresample   0. 17.104 /  0. 17.104
  libpostproc    52.  3.100 / 52.  3.100
Input #0, flac, from 'C418 - Minecraft - Volume Alpha - 01 Key.flac':
  Metadata:
    TITLE           : Key
    ARTIST          : C418
    DATE            : 2011
    ALBUM           : Minecraft - Volume Alpha
    track           : 1
  Duration: 00:01:05.00, bitrate: 311 kb/s
    Stream #0:0: Audio: flac, 44100 Hz, stereo, s16
    Stream #0:1: Video: png, rgb24, 700x700 [SAR 53520:53520 DAR 1:1], 90k tbr, 90k tbn, 90k tbc
    Metadata:
      comment         : Cover (front)
File 'C418 - Minecraft - Volume Alpha - 01 Key.opus' already exists. Overwrite ? [y/N] y
Output #0, ogg, to 'C418 - Minecraft - Volume Alpha - 01 Key.opus':
  Metadata:
    TITLE           : Key
    ARTIST          : C418
    DATE            : 2011
    ALBUM           : Minecraft - Volume Alpha
    track           : 1
    encoder         : Lavf55.19.104
    Stream #0:0: Audio: flac, 44100 Hz, stereo, s16, 128 kb/s
    Metadata:
      TITLE           : Key
      ARTIST          : C418
      DATE            : 2011
      ALBUM           : Minecraft - Volume Alpha
      TRACKNUMBER     : 1
      encoder         : Lavf55.19.104
Stream mapping:
  Stream #0:0 -> #0:0 (flac -> flac)
Press [q] to stop, [?] for help
size=    2038kB time=00:01:05.09 bitrate= 256.5kbits/s    
video:0kB audio:2028kB subtitle:0 global headers:0kB muxing overhead 0.499630%

[Carl Eugen Hoyos]

Is this problem not reproducible with current git head?

[Carl Eugen Hoyos]

Should be fixed in 2ccc6ff0 - thank you for the report!