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/Attributes/NamedArgumentTypeAttribute.cs

@@ -0,0 +1,11 @@
+using System;
+
+namespace Discord.Commands
+{
+    /// <summary>
+    /// Instructs the command system to treat command paramters of this type
+    /// as a collection of named arguments matching to its properties.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = true)]
+    public sealed class NamedArgumentTypeAttribute : Attribute { }
+}

src/Discord.Net.Commands/Attributes/OverrideTypeReaderAttribute.cs

@@ -1,5 +1,4 @@
 using System;
-
 using System.Reflection;
 
 namespace Discord.Commands
@@ -27,8 +26,8 @@ namespace Discord.Commands
     ///         => ReplyAsync(time);
     ///     </code>
     /// </example>
-    [AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false, Inherited = true)]
-    public class OverrideTypeReaderAttribute : Attribute
+    [AttributeUsage(AttributeTargets.Parameter | AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
+    public sealed class OverrideTypeReaderAttribute : Attribute
     {
         private static readonly TypeInfo TypeReaderTypeInfo = typeof(TypeReader).GetTypeInfo();