ffmpeg: add enc_time_base option

add a per-stream option for setting the encoder timebase.
the following values are allowed:
0 - for video, use 1/frame_rate, for audio use 1/sample_rate (this is
  the default)
-1 - match the input timebase (when possible)
>0 - set the timebase to provided number

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>