Multi-target .NET Standard 1.4 and 2.0

See guidance on [Cross-platform targeting](https://docs.microsoft.com/en-us/dotnet/standard/library-guidance/cross-platform-targeting#net-standard)

ColorCode.Core/ColorCode.Core.csproj

@@ -2,7 +2,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard1.4</TargetFramework>
+    <TargetFrameworks>netstandard1.4;netstandard2.0</TargetFrameworks>
     <RootNamespace>ColorCode</RootNamespace>
     <AssemblyName>ColorCode.Core</AssemblyName>
     <Title>ColorCode-Universal</Title>

ColorCode.HTML/ColorCode.HTML.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard1.4</TargetFramework>
+    <TargetFrameworks>netstandard1.4;netstandard2.0</TargetFrameworks>
     <RootNamespace>ColorCode</RootNamespace>
     <AssemblyName>ColorCode.HTML</AssemblyName>
     <Title>ColorCode.HTML</Title>