Added support for .NET Standard 2.0 (fixes #16) (#17)

Author: NightOwl888

src/LiquidTestReports.Core/LiquidTestReports.Core.csproj

@@ -2,7 +2,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <AssemblyName>LiquidTestReports.Core</AssemblyName>
-    <TargetFrameworks>netstandard2.1;net451</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;net451</TargetFrameworks>
     <RepositoryUrl>https://github.com/kurtmkurtm/LiquidTestReports</RepositoryUrl>
     <PackageProjectUrl>https://github.com/kurtmkurtm/LiquidTestReports</PackageProjectUrl>
     <PackageLicenseFile>LICENSE</PackageLicenseFile>

src/LiquidTestReports.Custom/LiquidTestReports.Custom.csproj

@@ -2,7 +2,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <AssemblyName>LiquidTestReports.Custom.TestLogger</AssemblyName>
-    <TargetFrameworks>netstandard2.1;net451</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;net451</TargetFrameworks>
     <RepositoryUrl>https://github.com/kurtmkurtm/LiquidTestReports</RepositoryUrl>
     <PackageProjectUrl>https://github.com/kurtmkurtm/LiquidTestReports</PackageProjectUrl>
     <PackageLicenseFile>LICENSE</PackageLicenseFile>

src/LiquidTestReports.Markdown/LiquidTestReports.Markdown.csproj

@@ -2,7 +2,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <AssemblyName>LiquidTestReports.Markdown.TestLogger</AssemblyName>
-    <TargetFrameworks>netstandard2.1;net451</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;net451</TargetFrameworks>
     <PackageLicenseFile>LICENSE</PackageLicenseFile>
     <PackageProjectUrl>https://github.com/kurtmkurtm/LiquidTestReports</PackageProjectUrl>
     <RepositoryUrl>https://github.com/kurtmkurtm/LiquidTestReports</RepositoryUrl>