ENH: output per-region counts (not means) for --by
currently, the output for each region specified via --by is the mean per-base coverage of the region. This is because that's what the machinery in `mosdepth` facilitates.

For many use-cases, users want the actual count of reads that overlap an interval. I have stubbed out  a way to do this that is fast enough to incorporate using [lapper](https://github.com/brentp/nim-lapper).

On my test-case with an exome, and a --by file with  1,195,764 intervals, this takes the run-time for mosdepth from 130 seconds to 140 seconds and user-time with 2 decompression threads from 50 seconds to 60 seconds. I think this is acceptable given the utility and the fact that `samtools bedcov` takes ~33 minutes on the same bam and bed file. So, this is **2:20 for mosdepth** and **33:30 for `samtools bedcov`**

With a bed file of 10 million 300-base windows (to simulate --by 300 because I haven't implemented this for integer window yet), the user time goes only up to 80 seconds for the same (exome bam). 

I would like to get feedback from @chapmanb and anyone else interested on how to expose this. Should this replace the output for `$prefix.regions.bed.gz` ? Or should it be a new file `$prefix.region-counts.bed.gz`? I would prefer to replace the output in `$prefix.regions.bed.gz` but are there any common use-cases for the current mean output?

note that this would leave all other output files unchanged.
