Allow commands to return a Task<RuntimeResult> (#466)

* Allow commands to return a Task<RuntimeResult>

This allows bot developers to centralize command result logic by
using result data whether the command as successful or not.

Example usage:
```csharp
var _result = await Commands.ExecuteAsync(context, argPos);
if (_result is RuntimeResult result)
{
    await message.Channel.SendMessageAsync(result.Reason);
}
else if (!_result.IsSuccess)
{
    // Previous error handling
}
```

The RuntimeResult class can be subclassed too, for example:
```csharp
var _result = await Commands.ExecuteAsync(context, argPos);
if (_result is MySubclassedResult result)
{
    var builder = new EmbedBuilder();
    for (var pair in result.Data)
    {
        builder.AddField(pair.Key, pair.Value, true);
    }
    await message.Channel.SendMessageAsync("", embed: builder);
}
else if (_result is RuntimeResult result)
{
    await message.Channel.SendMessageAsync(result.Reason);
}
else if (!_result.IsSuccess)
{
    // Previous error handling
}
```

* Make RuntimeResult's ctor protected

* Make RuntimeResult abstract

It never really made sense to have it instantiable in the first place,
frankly.

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

@@ -197,22 +197,34 @@ namespace Discord.Commands
 
             var createInstance = ReflectionUtils.CreateBuilder<IModuleBase>(typeInfo, service);
 
-            builder.Callback = async (ctx, args, map, cmd) => 
+            async Task<IResult> ExecuteCallback(ICommandContext context, object[] args, IServiceProvider services, CommandInfo cmd)
             {
-                var instance = createInstance(map);
-                instance.SetContext(ctx);
+                var instance = createInstance(services);
+                instance.SetContext(context);
+
                 try
                 {
                     instance.BeforeExecute(cmd);
+
                     var task = method.Invoke(instance, args) as Task ?? Task.Delay(0);
-                    await task.ConfigureAwait(false);
+                    if (task is Task<RuntimeResult> resultTask)
+                    {
+                        return await resultTask.ConfigureAwait(false);
+                    }
+                    else
+                    {
+                        await task.ConfigureAwait(false);
+                        return ExecuteResult.FromSuccess();
+                    }
                 }
                 finally
                 {
                     instance.AfterExecute(cmd);
                     (instance as IDisposable)?.Dispose();
                 }
-            };
+            }
+
+            builder.Callback = ExecuteCallback;
         }
 
         private static void BuildParameter(ParameterBuilder builder, System.Reflection.ParameterInfo paramInfo, int position, int count, CommandService service)
@@ -293,7 +305,7 @@ namespace Discord.Commands
         private static bool IsValidCommandDefinition(MethodInfo methodInfo)
         {
             return methodInfo.IsDefined(typeof(CommandAttribute)) &&
-                   (methodInfo.ReturnType == typeof(Task) || methodInfo.ReturnType == typeof(void)) &&
+                   (methodInfo.ReturnType == typeof(Task) || methodInfo.ReturnType == typeof(Task<RuntimeResult>)) &&
                    !methodInfo.IsStatic &&
                    !methodInfo.IsGenericMethod;
         }