Fragment length filtering
My colleagues and I often have to run multiple experiments on coverage within different fragment length ranges. To do this with `mosdepth` we currently have to subset the bam files first, for each experiment. 

Considering how much easier it would be to have a fragment length filter in mosdepth instead, I would like to request this option (similar request to https://github.com/brentp/mosdepth/issues/122 ). It would likely be as simple as an extra if-statement in `coverage()` next to the mapq filter? I don't know `nim` though, otherwise I would have given it a try.

I would want two new flags: `--min_length` and `--max_length` (or similar), where you can supply one or both.

Thanks a lot for this great tool!