Add demo clear-chat command

This command is very unoptimized (spawning 12 threads) and sometimes hits 429 errors, but it works until things get more serious. Also it's hardcoded to only delete 10 messages.

src/main/java/wtf/beatrice/hidekobot/HidekoBot.java

@@ -16,7 +16,8 @@ import java.util.List;
 public class HidekoBot
 {
     private static String botToken;
-    private static String standardInviteLink = "https://discord.com/oauth2/authorize?client_id=%userid%&scope=bot&permissions=8";
+    private static String standardInviteLink =
+            "https://discord.com/oauth2/authorize?client_id=%userid%&scope=bot&permissions=8";
     private static String botUserId;
     private static final String version = "0.0.1"; // we should probably find a way to make this consistent with Maven