Add defaults for mpeg2video encoder

[alexheretic]

mpeg2video: map --crf to ffmpeg -q and set default crf range to 2-30.

Usage example

$ ab-av1 crf-search -i vid.mp4 -e mpeg2video --max-encoded-percent 300
- crf 16 VMAF 83.56 (84%) (cache)
- crf 8 VMAF 91.74 (143%) (cache)
- crf 2 VMAF 97.33 (485%) (cache)
- crf 5 VMAF 94.43 (209%) (cache)
  00:00:00 crf 4 1/1 ##################################################### (eta 0s)
Encode with: ab-av1 encode -e mpeg2video -i vid.mp4 --crf 4

crf 4 VMAF 95.61 predicted video stream size 52.16 MiB (259%) taking 1 second