bigwig output
it will be useful to have bigwig output from mosdepth. this is my tentative plan; feedback by users would be appreciated.

mosdepth will remain unchanged, except that when per-base output is requested, it will output bigwig instead of bed.gz--there will be no option to request bigwig and it will not be written for quantized/regions/thresholds. 
quantized/regions/thresholds can be quickly derived from bigwig using [bigwig](https://github.com/brentp/bigwig-nim/) or [pybigwig](https://github.com/deeptools/pyBigWig). I will be building out bigwig to support all operations such that a user can run `mosdepth` once to get per-base bigwig output and then quickly get quantized/regions/thresholds from the bigwig.

mosdepth will still support reporting quantized/regions/thresholds directly, but the idea is that once these can be pull from the per-base bigwig nearly instantly, there's less need to report them.

if users foresee any problems with removing per-base.bed.gz in favor of per-base.bed.bw, please let me know.