Add original XSD elements to INamingProvider methods

[aparshin]

Fixes #293.

Note that this is not a backward-compatible improvement (INamingProvider interface is changed). Unfortunately, I don't have any good ideas on how to make it backward-compatible (except a separate interface and new configuration option, which looks pretty silly to me).

[codecov]

Codecov Report

Merging #295 (f1bc752) into master (9c6f1dc) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #295   +/-   ##
=======================================
  Coverage   95.34%   95.34%           
=======================================
  Files          19       19           
  Lines        3286     3286           
  Branches      478      478           
=======================================
  Hits         3133     3133           
  Misses         90       90           
  Partials       63       63           

Impacted Files	Coverage Δ
XmlSchemaClassGenerator/ModelBuilder.cs	96.90% <100.00%> (ø)
XmlSchemaClassGenerator/NamingProvider.cs	100.00% <100.00%> (ø)

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[mganss]

Thanks!