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

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 67.82%. Comparing base (8a26e26) to head (c43cd24).
⚠️ Report is 121 commits behind head on master.

Files with missing lines	Patch %	Lines
src/IterTools.jl	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            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              

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