Change to es6 class syntax

[joehand]

As suggested in #232.

[codecov-io]

Codecov Report

Merging #234 into master will decrease coverage by 0.37%.
The diff coverage is 90.47%.

@@            Coverage Diff             @@
##           master     #234      +/-   ##
==========================================
- Coverage    85.9%   85.52%   -0.38%     
==========================================
  Files           7        7              
  Lines         305      297       -8     
==========================================
- Hits          262      254       -8     
  Misses         43       43

Impacted Files	Coverage Δ
index.js	100% <100%> (ø)	⬆️
dat.js	90.36% <90.36%> (-0.85%)	⬇️

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