Flesh out allowlist handling

Addresses comments from https://github.com/LOOHP/Limbo/pull/57#issuecomment-1304543589

In particular:

* Functionality now matches other server types

* only enforce the allowlist if a new "enforce-allowlist"
  boolean in server.properties is set to true

* Loads and process the allowlist only once when the server starts (or
  the reload command is executed), instead of every time a user connects.

* Add a new command & associated permissions "allowlist reload"
  to reload the allowlist

src/main/java/com/loohp/limbo/commands/DefaultCommands.java

@@ -165,6 +165,20 @@ public class DefaultCommands implements CommandExecutor, TabCompletor {
 			}
 			return;
 		}
+		if (args[0].equalsIgnoreCase("allowlist")) {
+			if (sender.hasPermission("limboserver.allowlist")) {
+				if (args.length != 2) {
+					sender.sendMessage(ChatColor.RED + "Invalid usage!");
+				} else if (!args[1].equalsIgnoreCase("reload")) {
+					sender.sendMessage(ChatColor.RED + "Invalid usage!");
+				} else {
+					Limbo.getInstance().reloadAllowlist();
+				}
+			} else {
+				sender.sendMessage(ChatColor.RED + "You do not have permission to use that command!");
+			}
+			return;
+		}
 	}
 	
 	@Override