support negative k for k-subsets

[stev47]

We have binomial(n, k)=0 for k<0 and k>n.
This makes subsets comply accordingly instead of creating wrong iterators or throwing errors.

[codecov-io]

Codecov Report

Merging #63 into master will increase coverage by 0.25%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
+ Coverage   67.57%   67.82%   +0.25%     
==========================================
  Files           1        1              
  Lines         256      258       +2     
==========================================
+ Hits          173      175       +2     
  Misses         83       83

Impacted Files	Coverage Δ
src/IterTools.jl	67.82% <66.66%> (+0.25%)	⬆️

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