Add support for passing tune parameter to aomenc

[0bugs]

Currently, the ffmpeg libaom encoder plugin does not allow passing any tune parameters (
​https://github.com/FFmpeg/FFmpeg/blob/master/libavcodec/libaomenc.c#L1063-L1100 )

This is considerably important as the latest vmaf tune aomenc got provides considerable bitrate reductions (37%) and thus makes it the one of the most important parameters available: ​https://aomedia.googlesource.com/aom/+/615dc24579d531cb3a2c9627ab25a3026f9e2b47