Only show actual available codecs in codecs list

[cessen]

Right now when the user uses the --codecs flag is just shows all of our pre-selected codecs. However, not all of those codecs are guaranteed to be supported by a given build of ffmpeg or avconv. This will be very confusing to the user if they try to use those codecs.

Let's change it so that the script actually figures out which of the curated codecs are supported, and only shows those.

[techtonik]

And I would be very interested in ability to write directly to .gif.