Feature Request: add --vmaf option in NVenc

[Dan64]

Hello,

   NVenc has the option to calculate the Netflix quality metric "vmaf" as described in https://github.com/rigaya/NVEnc/blob/mas...ions.en.md

   Even if this option is available only for Windows x64 and could represent a bottleneck for the encoding process, it could be very useful in some circumstances.

Thanks