Make unviewable page URI configurable via options

[cdrini]

Demo here: https://deploy-preview-1324--lucid-poitras-9a1249.netlify.app/bookreaderdemo/demo-preview-pages

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 40.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 69.26%. Comparing base (2d19d55) to head (4f094ee).

Files	Patch %	Lines
src/BookReader/BookModel.js	40.00%	3 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1324      +/-   ##
==========================================
- Coverage   69.30%   69.26%   -0.05%     
==========================================
  Files          59       59              
  Lines        5082     5085       +3     
  Branches     1069     1070       +1     
==========================================
  Hits         3522     3522              
- Misses       1533     1536       +3     
  Partials       27       27              

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