Add priority/niceness for re-encoding in videoFileClip.write_videofile() as argument

[serialjaywalker]

We already have a option for thread count, but this seems a naive solution. Niceness/priority would be useful for many homelabs/NAS machines. I would like to use my NAS/pi to run re-encoding but do so at a lower priority to other tasks. Simply checking for core counts (already implemented) and using less than the max core count -N wouldn't solve it out-of-the-box. We could add the option for niceness/priority to videoFileClip.write_videofile() in its call to ffmpeg so that re-encoding/resizing/etc could perform as background task while others get priority.

This is useful for archival purposes such as converting h264 to h465/hevc or another space-saving format on low/mid powered systems and multi-use systems, such as a NAS.