Fix: Polyfill CustomEvent constructor

[cdrini]

This was causing IE11 to brick. Apparently not supported by core-js :(
zloirock/core-js#354

[codecov]

Codecov Report

Merging #444 into master will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #444      +/-   ##
==========================================
+ Coverage   63.09%   63.15%   +0.06%     
==========================================
  Files          34       34              
  Lines        3636     3642       +6     
  Branches      687      688       +1     
==========================================
+ Hits         2294     2300       +6     
  Misses       1084     1084              
  Partials      258      258              

Impacted Files	Coverage Δ
src/js/BookReader.js	66.45% <100.00%> (+0.02%)	⬆️
src/js/BookReader/utils.js	84.21% <100.00%> (+2.39%)	⬆️

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[cdrini]

Tested on IE11 ✅