Added netstandard2.0 target framework

[kashifsoofi]

Added netstandard2.0 target framework

[StefH]

AppVeyor build fails because .net core 1.1 is used to build the source (dotnet-1.1.1-sdk-win-x64.exe), can you please also update the https://github.com/StefH/XPath2.Net/blob/master/appveyor.yml so that the dotnet 2.0 is used ?

[codecov[bot]]

Codecov Report

Merging #8 into master will increase coverage by 0.07%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master       #8      +/-   ##
==========================================
+ Coverage   17.35%   17.42%   +0.07%     
==========================================
  Files         130      130              
  Lines       14800    14800              
  Branches     2025     2025              
==========================================
+ Hits         2568     2579      +11     
+ Misses      11983    11976       -7     
+ Partials      249      245       -4

Impacted Files	Coverage Δ
src/XPath2/Iterator/AxisNodeIterator.cs	65.3% <0%> (-4.09%)	:arrow_down:
src/XPath2/XPath2Item.cs	11.37% <0%> (-1.8%)	:arrow_down:
src/XPath2/XPath2Convert.cs	4% <0%> (-0.88%)	:arrow_down:
src/XPath2/CoreFuncs.cs	12.24% <0%> (+0.27%)	:arrow_up:
src/XPath2/Iterator/SequentialAxisNodeIterator.cs	88.57% <0%> (+5.71%)	:arrow_up:
src/XPath2.Extensions/Json2XmlUtils.cs	54.71% <0%> (+30.18%)	:arrow_up:

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