Add support for cleanup function

[ganemone]

Fixes #75

[codecov]

Codecov Report

Merging #76 into master will decrease coverage by 1.48%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #76      +/-   ##
==========================================
- Coverage   86.95%   85.47%   -1.49%     
==========================================
  Files           4        4              
  Lines         115      117       +2     
  Branches       20       20              
==========================================
  Hits          100      100              
- Misses          8       10       +2     
  Partials        7        7

Impacted Files	Coverage Δ
src/browser.js	84.84% <0%> (-5.48%)	⬇️

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