Support streaming from cloud storage buckets
Hi, 

Given that mosdepth can be installed from source with independent htslib, and htslib supports streaming from cloud storage buckets, I'm wondering how difficult it would be, for mosdepth to support streaming from cloud storage directly.

E.g. something like the following (using Google bucket as example):

```
mosdepth -n --fast-mode --by 500 sample.wgs gs://path-to-my.bam
```

Thank you
Steve