Add helper to broadcast binary ops with numba

[xhochy]

Checklist

• Added a CHANGELOG.md entry

[codecov]

Codecov Report

Merging #208 (0ce7d4b) into master (4ed9059) will increase coverage by 0.32%.
The diff coverage is 96.85%.

@@            Coverage Diff             @@
##           master     #208      +/-   ##
==========================================
+ Coverage   84.60%   84.93%   +0.32%     
==========================================
  Files          32       32              
  Lines        4553     4679     +126     
==========================================
+ Hits         3852     3974     +122     
- Misses        701      705       +4     

Impacted Files	Coverage Δ
fletcher/algorithms/string.py	90.22% <94.59%> (+0.62%)	⬆️
tests/test_text.py	98.52% <100.00%> (+0.15%)	⬆️

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[xhochy]

Tests are all green now.