Fix an issue that transcoder couldn't handle a sequence pattern output

FFMpeg's image2 file format allows output filename to use a sequence pattern, like "screenshot%d.jpg".
cf. http://www.ffmpeg.org/ffmpeg-formats.html#image2-2
Example usage in command line
```
ffmpeg -i input.mp4 -ss 1 -r 1 -f image2 screenshot%d.jpg
```
and I tried this code
```
movie.transcode("screenshot%d.jpg", "-ss 1 -r 1 -f image2")
```
The issue is that this gem's transcoder couldn't validate it.