feature: Add a way to invoke a command specifying optional values by name (#1123)

* Add NamedArgumentTypeAttribute

* Add NamedArgumentTypeReader

* Fix superflous empty line.

* Fix logic for quoted arguments

* Throw an exception with a tailored message.

* Add a catch to wrap parsing/input errors

* Trim potential excess whitespace

* Fix an off-by-one

* Support to read an IEnumerable property

* Add a doc

* Add assertion for the collection test

test/Discord.Net.Tests/Discord.Net.Tests.csproj

@@ -3,6 +3,7 @@
     <OutputType>Exe</OutputType>
     <RootNamespace>Discord</RootNamespace>
     <TargetFramework>netcoreapp1.1</TargetFramework>
+    <DebugType>portable</DebugType>
     <PackageTargetFallback>$(PackageTargetFallback);portable-net45+win8+wp8+wpa81</PackageTargetFallback>
   </PropertyGroup>
   <ItemGroup>
@@ -23,8 +24,8 @@
     <PackageReference Include="Akavache" Version="5.0.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.7.2" />
     <PackageReference Include="Newtonsoft.Json" Version="11.0.2" />
-    <PackageReference Include="xunit" Version="2.3.1" />
-    <PackageReference Include="xunit.runner.visualstudio" Version="2.3.1" />
-    <PackageReference Include="xunit.runner.reporters" Version="2.3.1" />
+    <PackageReference Include="xunit" Version="2.4.0" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.4.0" />
+    <PackageReference Include="xunit.runner.reporters" Version="2.4.0" />
   </ItemGroup>
 </Project>