Allow using an abundance file with --draft

[apcamargo]

This is a very crude solution to allow using --abundance_file and --coverage together with --draft. The code should probably be moved to the abundance.draft function.

Solves #83

[HadrienG]

Thanks for the contribution! I had already started with my own solution and made it work with the improved --coverage and --coverage_file option, but I appreciate the help.

/Hadrien

[apcamargo]

Thanks @HadrienG!

This was just some quick and dirty code that I wrote because I needed that functionality at the time. I'm happy you got the time to do a proper implementation!