Feature Request: Add threshold for Whole Genome
Hi!,

Currently the threshold feature can output the chromosome/scaffold wise threshold outcome for every/selected chromosome.

Currently it looks like this:

````
#chrom  start   end     region  1X      10X     20X     30X
1       0       197608386       unknown 195634794       183535477       51086214        2596469
2       0       149682049       unknown 148581500       139670841       37081610        1436371
````

I believe that we can add a line for the whole genome as well like this:

````
#chrom  start   end     region  1X      10X     20X     30X
Total  0       sum(1+2)         unknown value_1X         value_10X       value_20X       value_30X
1       0       197608386       unknown 195634794       183535477       51086214        2596469
2       0       149682049       unknown 148581500       139670841       37081610        1436371
````
