Support "guarded expressions" when concatting String values.

e.g. {"[" + {n} + " " + {s00e00} + "]"}

https://www.filebot.net/forums/viewtopic.php?f=8&t=10719

source/net/filebot/format/ExpressionFormatMethods.java

@@ -676,6 +676,11 @@ public class ExpressionFormatMethods {
 		return '-' + self;
 	}
 
+	public static String plus(String self, Closure closure) {
+		Object value = call(closure);
+		return value == null ? self : self + value;
+	}
+
 	private ExpressionFormatMethods() {
 		throw new UnsupportedOperationException();
 	}