add video options of transcoding by file size

[andyrozman]

ffmpeg which is used by Arista, has also option of getting output file with specific size. While this is not so simple process (you can't just specify size on Command line, but you need to calculate bitrate and then use this on command line) it's feature many linux users will like...
I always target specific size for files I transcode, which is only reason I am always running VMWare with Windows on my Linux machine. I think this would be nice feature to have with Arista, and Arista will be probably first Opensorce software that will have this.., (I checked all alternatives for Xvid4PSP which is software I use under windows).

Here is link to exact instructions, how to do this:
https://trac.ffmpeg.org/wiki/x264EncodingGuide
See section "Two pass"