Make reduceSet option allow a custom ReduceSet object.

[cdrini]

Besides the two default named options, pow2 and integer, add another option for a custom ReduceSet object.

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (5f2a248) 69.30% compared to head (fbb99d9) 69.29%.

Files	Patch %	Lines
src/BookReader.js	50.00%	3 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1317      +/-   ##
==========================================
- Coverage   69.30%   69.29%   -0.01%     
==========================================
  Files          59       59              
  Lines        5082     5084       +2     
  Branches     1069     1070       +1     
==========================================
+ Hits         3522     3523       +1     
- Misses       1533     1534       +1     
  Partials       27       27              

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