Switch netcoreapp target to netstandard2.0 (#284)

Fixes #283 

Instead of adding another target framework to the list, switch from netcoreapp2.0 to netstandard2.0, as ns2.0 is a subset of netcoreapp.

Terminal.Gui/Terminal.Gui.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>net461;netcoreapp2.0</TargetFrameworks>
+    <TargetFrameworks>net461;netstandard2.0</TargetFrameworks>
     <RootNamespace>Terminal.Gui</RootNamespace>
     <AssemblyName>Terminal.Gui</AssemblyName>
     <DocumentationFile>bin\Release\Terminal.Gui.xml</DocumentationFile>