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

src/Discord.Net.Commands/Builders/ModuleClassBuilder.cs

@@ -280,7 +280,7 @@ namespace Discord.Commands
             }
         }
 
-        private static TypeReader GetTypeReader(CommandService service, Type paramType, Type typeReaderType, IServiceProvider services)
+        internal static TypeReader GetTypeReader(CommandService service, Type paramType, Type typeReaderType, IServiceProvider services)
         {
             var readers = service.GetTypeReaders(paramType);
             TypeReader reader = null;