Add CommandServiceConfig, DefaultRunMode

This adds an (optional) CommandServiceConfig, as well as a DefaultRunMode for commands.

This resolves #368 (for commands where a RunMode is not explicitly specified, a custom default value should be used)

src/Discord.Net.Commands/CommandServiceConfig.cs

@@ -0,0 +1,8 @@
+namespace Discord.Commands
+{
+    public class CommandServiceConfig
+    {
+        /// <summary> The default RunMode commands should have, if one is not specified on the Command attribute or builder. </summary>
+        public RunMode DefaultRunMode { get; set; } = RunMode.Mixed;
+    }
+}