Add SWCommandUtils.addMapper

SpigotCore_Main/src/de/steamwar/command/SWCommandUtils.java

@@ -131,4 +131,9 @@ class SWCommandUtils {
         }
     }
 
+    public static <K> void addMapper(Class<K> clazz, Function<String, K> mapper) {
+        if (MAPPER_FUNCTIONS.containsKey(clazz)) return;
+        MAPPER_FUNCTIONS.put(clazz, mapper);
+    }
+
 }