median and summary file

• this release adds a new *.mosdepth.summary.txt output file added by @danielecook. It reports some statistics for each chromosome.
• it also adds a --median flag to be applied to the regions given in --by. The default is to use mean. This mode is recommended for more stable estimate of depth.
• fix for #54 for quantize.

To get started, use:

wget https://github.com/brentp/mosdepth/releases/download/v0.2.6/mosdepth && chmod +x ./mosdepth && ./mosdepth -h

That is the (recommended) static binary. To use one that depends on your local htslib (libhts.so), download this binary