Code cleanup.

src/main/java/com/github/imdmk/spenttime/SpentTime.java

@@ -130,8 +130,12 @@ public SpentTime(Plugin plugin) {
 
         /* PlaceholderAPI */
         if (this.server.getPluginManager().getPlugin("PlaceholderAPI") != null) {
-            this.placeholderRegistry = new PlaceholderRegistry(plugin.getDescription());
-            this.placeholderRegistry.registerAll();
+            this.placeholderRegistry = new PlaceholderRegistry();
+
+            Stream.of(
+                    new SpentTimeFormattedPlaceholder(plugin.getDescription()),
+                    new SpentTimePlaceholder(plugin.getDescription())
+            ).forEach(this.placeholderRegistry::register);
         }
 
         /* Update check */

src/main/java/com/github/imdmk/spenttime/placeholder/PlaceholderRegistry.java

@@ -1,26 +1,27 @@
 package com.github.imdmk.spenttime.placeholder;
 
-import com.github.imdmk.spenttime.placeholder.implementation.SpentTimeFormattedPlaceholder;
-import com.github.imdmk.spenttime.placeholder.implementation.SpentTimePlaceholder;
-import org.bukkit.plugin.PluginDescriptionFile;
+import me.clip.placeholderapi.expansion.PlaceholderExpansion;
+
+import java.util.HashSet;
+import java.util.Set;
 
 public class PlaceholderRegistry {
 
-    private final SpentTimeFormattedPlaceholder spentTimeFormattedPlaceholder;
-    private final SpentTimePlaceholder spentTimePlaceholder;
+    private final Set<PlaceholderExpansion> placeholderExpansions = new HashSet<>();
+
+    public void register(PlaceholderExpansion placeholder) {
+        this.placeholderExpansions.add(placeholder);
 
-    public PlaceholderRegistry(PluginDescriptionFile pluginDescriptionFile) {
-        this.spentTimeFormattedPlaceholder = new SpentTimeFormattedPlaceholder(pluginDescriptionFile);
-        this.spentTimePlaceholder = new SpentTimePlaceholder(pluginDescriptionFile);
+        placeholder.register();
     }
 
-    public void registerAll() {
-        this.spentTimeFormattedPlaceholder.register();
-        this.spentTimePlaceholder.register();
+    public void unregister(PlaceholderExpansion placeholderExpansion) {
+        this.placeholderExpansions.remove(placeholderExpansion);
+
+        placeholderExpansion.unregister();
     }
 
     public void unregisterAll() {
-        this.spentTimeFormattedPlaceholder.unregister();
-        this.spentTimePlaceholder.unregister();
+        this.placeholderExpansions.forEach(this::unregister);
     }
 }