Fix support for multi-byte characters

[adrg]

• Fix support for multibyte characters throughout the package.
• Improve test cases.

Addresses issue #18.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (7ba2633) 0.00% compared to head (f86c21b) 100.00%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@              Coverage Diff              @@
##           master       #27        +/-   ##
=============================================
+ Coverage        0   100.00%   +100.00%     
=============================================
  Files           0        13        +13     
  Lines           0       466       +466     
=============================================
+ Hits            0       466       +466     

Files	Coverage Δ
internal/stringutil/stringutil.go	100.00% <100.00%> (ø)
metrics/jaro.go	100.00% <100.00%> (ø)
metrics/levenshtein.go	100.00% <100.00%> (ø)

... and 10 files with indirect coverage changes

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