function nthcomb to match nthperm

I needed to sample the nth lexicographic combination of from a collection rather than nth permutation for some internal projects. I'm wondering if there's interest to have this functionality as part of `Combinatorics.jl` to match `nthperm()`. I've extracted the code to do this from the other project and made this repo https://github.com/ajloza/NthComb.jl
Right now it's not using the nomenclature of `nthperm` for variable names, so if there is interest, I'd refactor it and try to match the api. Apologies if I've missed this functionality or an issue addressing it