src: Replace var with let

[lholmquist]

This replaces the usage of var with let to make the code more consistent. This was something i noticed while working on #71

[codecov]

Codecov Report

Merging #72 into master will not change coverage.
The diff coverage is 58.33%.

@@           Coverage Diff           @@
##           master      #72   +/-   ##
=======================================
  Coverage   57.14%   57.14%           
=======================================
  Files          18       18           
  Lines         448      448           
=======================================
  Hits          256      256           
  Misses        192      192

Impacted Files	Coverage Δ
lib/format-pretty.js	9.52% <0%> (ø)	⬆️
bin/cmd.js	29.21% <0%> (ø)	⬆️
lib/rules/subsystem.js	100% <100%> (ø)	⬆️
lib/rules/fixes-url.js	96.29% <100%> (ø)	⬆️
lib/rules/metadata-end.js	100% <100%> (ø)	⬆️
lib/rules/pr-url.js	100% <100%> (ø)	⬆️
lib/rules/line-length.js	100% <100%> (ø)	⬆️

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