tmp dir
Hi, 
I have an error that occurs randomly with mosdepth
```
[mosdepth] could not open file:/tmp//A1127C96_markdup_mosdepth_output.mosdepth.global.dist.txt
../root/.nimble/pkgs/hts-0.2.7/hts/bgzf.nim(52) open
```
I guess that, at some point, it tries to write a tmp file, and to reload it later.
Not sure it comes from mosdepth itself ...
However it would great to be able to modify the tmp folder ...
Do you think you can add such option !?
Many thanks
Nicolas
