Request pool manager options

[JamesAPetts]

Cornerstone imageLoaders registered to cornerstone can take options which may specific to the loader themselves.

This PR adds an argument to the request pool manager to pass options down options to its managed cornerstone.loadImage and cornerstone.loadAndCacheImage calls.

These options are merged with the default options request pool manager provides.

[swederik]

Options shouldn't overwrite priority and type, otherwise it's fine.

[codecov]

Codecov Report

Merging #1324 into master will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1324      +/-   ##
==========================================
- Coverage   18.94%   18.94%   -0.01%     
==========================================
  Files         286      286              
  Lines        8671     8672       +1     
  Branches     1486     1486              
==========================================
  Hits         1643     1643              
- Misses       5815     5816       +1     
  Partials     1213     1213              

Impacted Files	Coverage Δ
src/requestPool/requestPoolManager.js	21.25% <0.00%> (-0.27%)	⬇️

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[dannyrb]

🎉 This PR is included in version 5.1.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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