Output only to two decimal places in $prefix.mosdepth.dist.txt
The current version of mosdepth (downloaded as a binary release) outputs cumulative distribution information only to two decimal places and has a lot of rows with 0.00 as a result. I noticed that an older version (downloaded Jan 4, 2018) output to 4 decimal places. E.g.:

#old version - $prefix.mosdepth.dist.txt:
```
chr1  591  0.0001  
chr1  590  0.0001  
chr1  589  0.0001  
...
total	2	0.9944
total	1	0.9978
total	0	1.0000
```

#current version - $prefix.mosdepth.dist.txt:
```
chr1  591  0.00  
chr1  590  0.00  
chr1  589  0.00
...
total	2	0.99
total	1	1.00
total	0	1.00
```

Please bring back the 4 decimal places in future releases. Thanks for this awesome tool!