[EXPORT] Allow access to FFmpeg command line options

[cgvirus]

Hi, as we have to do lots of optimization tricks with ffmpeg it would be nice to have at least a ffmpeg command string/argument support for technical people. Or better, making and saving render profile. Side Note: Prores is a bit confusing here. Is it using prores_ks or just prores? Also what about choosing profile like 0,1,2 or 3?

If it can be done in .ove, that will be enough as well for now. Please let me know. Thanks.

[DaniSeeh]

I believe Olive is presently in the process of disconnecting itself a little bit from FFmpeg, so I don't know if @itsmattkc intends to keep it tightly integrated enough for this to work. I don't know if the Abstraction Layer being written only applies to previews or final encodings too.