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/Attributes/CommandAttribute.cs

@@ -6,7 +6,7 @@ namespace Discord.Commands
     public class CommandAttribute : Attribute
     {
         public string Text { get; }
-        public RunMode RunMode { get; set; } = RunMode.Sync;
+        public RunMode RunMode { get; set; }
 
         public CommandAttribute()
         {