Add suport for .avs (avisynth files)

the Great windows app Avisynth works fine on Linux (under wine) and they are in the middle or porting it right now.
On windows I used to use Adobe Premiere because it could open avisynth scripts (using a plugin) - which meant I could do all kinds of adjustments and also open files types that Premiere would not ordinarily be able to open. Not many video apps support avisynth scrpt which is a great pity as it is so useful.