Add Fuzzy Jacard similarity

[koheiw]

for #42

[codecov[bot]]

Codecov Report

Merging #43 (277e1c1) into master (b349c4a) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #43      +/-   ##
==========================================
+ Coverage   99.50%   99.51%   +0.01%     
==========================================
  Files           4        4              
  Lines         401      416      +15     
==========================================
+ Hits          399      414      +15     
  Misses          2        2              

Files	Coverage Δ
R/proxy.R	99.12% <100.00%> (ø)
src/pair.cpp	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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