accept predefined boxes in goals

src/lib.rs

@@ -159,6 +159,7 @@ impl Level {
                         }
                         'X' => Ok(Cell::Wall),
                         '*' => Ok(Cell::Box),
+                        '!' => Ok(Cell::Goal(true)),
                         'g' => Ok(Cell::Goal(false)),
                         '.' => Ok(Cell::None),
                         _ => Err(ReadError::BadSymbol),