Add tests for attachColorLine and refactor

[tilfin]

No description provided.

[codecov-io]

Codecov Report

Merging #49 into master will increase coverage by 3.73%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
+ Coverage    83.9%   87.64%   +3.73%     
==========================================
  Files           7        7              
  Lines         261      259       -2     
==========================================
+ Hits          219      227       +8     
+ Misses         42       32      -10

Impacted Files	Coverage Δ
src/lib/content.js	96.42% <100%> (+8.7%)	⬆️

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