Add .netstandard 2.0 target (#44)

Author: daningalla

src/Vertical.CommandLine.csproj

@@ -1,6 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>netstandard2.1;net5.0</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;netstandard2.1;net5.0</TargetFrameworks>
+    <LangVersion>8.0</LangVersion>
     <RootNamespace />    
     <TreatWarningsAsErrors>True</TreatWarningsAsErrors>
     <SignAssembly>true</SignAssembly>
@@ -28,4 +29,8 @@
   <ItemGroup>
     <None Include="../assets/package-logo.png" Pack="true" Visible="false" PackagePath="" />
   </ItemGroup>
+  
+  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
+    <PackageReference Include="Microsoft.Bcl.HashCode" Version="1.1.1" />
+  </ItemGroup>
 </Project>

src/Vertical/CommandLine/CommandLineApplication.cs

@@ -94,7 +94,7 @@ public static void ShowHelp(ICommandLineConfiguration configuration,
                 ? new[] {helpToken}
                 : new[] {command, helpToken};
 
-            Run(configuration, args);
+            Run(configuration, args!);
         }
     }
 }