--d4 option unavailble in 0.3.4
Hi Brent,

I installed mosdepth via conda, and seems like the `--d4` is unavailable in the above version. Here is the screenshot
```
(mosdepth)[am]$ mosdepth --version 
mosdepth 0.3.4
(mosdepth)[am]$ mosdepth -t 8 --d4  -f genomes/hg38.fa 213470-mosdepth_perbase_d4 213470.cram
Usage: mosdepth [options] <prefix> <BAM-or-CRAM>
error parsing arguments
```

Thanks,
Aarthi