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/Info/CommandInfo.cs

@@ -37,7 +37,7 @@ namespace Discord.Commands
             Summary = builder.Summary;
             Remarks = builder.Remarks;
 
-            RunMode = builder.RunMode;
+            RunMode = builder.RunMode ?? service._defaultRunMode;
             Priority = builder.Priority;
             
             if (module.Aliases.Count != 0)