operon: allow setting faststart

[ghost]

FFmpeg can set faststart via:

ffmpeg alfa.m4a -c copy -movflags faststart bravo.m4a

TagEditor can set faststart via:

tageditor set --index-pos front --force --files alfa.m4a

in addition TagEditor can detect faststart via:

tageditor info --files bravo.m4a

Operon seems to have no way to set or even detect faststart
(moov before mdat).

[lazka]

Why do you need it?

[ghost]

@lazka certainly players (car stereo) will not even open M4A files unless they have faststart