Add shouldProcessNamespaces decoder property

[MaxDesiatov]

This allows us to implement a fix for CoreOffice/CoreXLSX#41

[codecov]

Codecov Report

Merging #81 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #81      +/-   ##
==========================================
+ Coverage   74.65%   74.74%   +0.08%     
==========================================
  Files          37       37              
  Lines        1752     1758       +6     
==========================================
+ Hits         1308     1314       +6     
  Misses        444      444

Impacted Files	Coverage Δ
Sources/XMLCoder/Decoder/XMLDecoder.swift	76.22% <100%> (+0.19%)	⬆️
Sources/XMLCoder/Auxiliaries/XMLStackParser.swift	93.1% <100%> (+0.42%)	⬆️

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