Add (optional) ffmpeg support

Here's a pull request for your consideration...

I was trying to setup a linux based NAS (x86) machine that I could push my video collection to, and then overnight it would process any outstanding queued items. The main issue that I had was that Handbrake was a giant pain to try and build for my specific setup. Although you can build it without a number of dependencies to get the command line version, it still required a significant amount of components. It was much easier to simply build ffmpeg which is arguably more powerful a command line tool, than Handbrake.

These changes add support for using ffmpeg instead of handbrake, although the default remains handbrake. The example settings are also similar to what's present for handbrake, e.g. copy all audio/subtitle streams, but encode the video using libx264 (medium preset with constant quality of 20). 

If you are interested in including these changes, and have any review comments, just let me know and i'll fix them up...