feat: per base in BED regions only?
Hi Brent,

Thanks for making such a great tool!

I've been using `-by` and I am curious if it would be possible to output per-base depth in the BED regions? 

My current workflow is to output the per-base depth then subset it to my regions of interest. My thought is that doing the above would maybe give some performance boost since mosdepth wont have to write every base. 

Let me know your thoughts, if you're open to a PR I can take a look at implementing this. 

Best,
Cade
