Feature request: Plot the chromosome lengths in covplot()

[amizeranschi]

This is not really an issue, but a feature request.

It would be very helpful to add an option to covpot() to represent the chromosomes and their lengths, as well. I mean, something like this: https://raw.githubusercontent.com/eweitz/ideogram/master/examples/vanilla/ideogram_histogram_all_human_genes.png

Given that some chromosomes are shorter than others, it is currently not possible to see if shorter chromosomes are fully covered in peaks, or if there are regions at their telomeres where there are no peaks present in the data. This could be some useful knowledge to gain from this plot.