Override audio settings for dolby surround 2.0 tracks

When transcoding an older DVD of a famous TV series, the audio track is in AC3 2.0 dolby surround. When the script autodetects this audio, it transforms it into AAC stereo without any fallback for having the sound copied as is.

The option `--force-encoder ENCODER` will undo all the audio-detect magic that happens in the transcode video script and apply the `ENCODER` option instead.

For example, this allows me to call the script like this:

``` bash
transcode-video.sh --force-encoder copy:ac3 --single --mkv --add-subtitle 1 --title 1 --output The\ X-Files\ S01E01.mkv /Volumes/XFILES_DISC1
```
And get the original soundtrack of the DVD.