Add Shop interface

src/main/java/com/alttd/playershops/shop/AbstractShop.java

@@ -1,5 +1,6 @@
 package com.alttd.playershops.shop;
 
+import com.alttd.playershops.api.Shop;
 import lombok.Getter;
 import lombok.Setter;
 import org.bukkit.Bukkit;
@@ -10,7 +11,7 @@ import org.bukkit.inventory.ItemStack;
 
 import java.util.UUID;
 
-public abstract class AbstractShop {
+public abstract class AbstractShop implements Shop {
 
     @Getter
     private int id;
@@ -38,6 +39,8 @@ public abstract class AbstractShop {
     @Getter @Setter
     private long lastTransaction;
 
+    protected boolean initialized;
+
     AbstractShop(Location signLocation, UUID uuid, double price, int amount) {
         this.signLocation = signLocation;
         if (signLocation != null) {
@@ -67,4 +70,8 @@ public abstract class AbstractShop {
         }
         return ChatColor.RED + "[CLOSED]";
     }
+
+    public boolean isInitialized() {
+        return initialized;
+    }
 }