Add .NET Core 2.2 and .NET Core 3.0 support in BenchmarkDotNet.Tool

Author: AndreyAkinshin

src/BenchmarkDotNet.Tool/BenchmarkDotNet.Tool.csproj

@@ -2,7 +2,7 @@
   <Import Project="..\..\build\common.props" />
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>netcoreapp2.1</TargetFramework>
+    <TargetFrameworks>netcoreapp2.1;netcoreapp2.2;netcoreapp3.0</TargetFrameworks>
     <IsPackable>true</IsPackable>
     <PackAsTool>true</PackAsTool>
     <ToolCommandName>dotnet-benchmark</ToolCommandName>