Add Autoload to Module Attribute

[UNTESTED] Adds an optional parameter to the Module attribute, "autoload", which defaults to true.

Specifies whether or not the assembly crawler should load this module.

src/Discord.Net.Commands/CommandService.cs

@@ -174,7 +174,7 @@ namespace Discord.Commands
                 {
                     var typeInfo = type.GetTypeInfo();
                     var moduleAttr = typeInfo.GetCustomAttribute<ModuleAttribute>();
-                    if (moduleAttr != null)
+                    if (moduleAttr != null && moduleAttr.Autoload)
                     {
                         var moduleInstance = ReflectionUtils.CreateObject(typeInfo);
                         modules.Add(LoadInternal(moduleInstance, moduleAttr, typeInfo));