Added layout hints config option to package diagrams

src/config/config.cc

@@ -409,6 +409,8 @@ template <> struct convert<package_diagram> {
         if (!decode_diagram(node, rhs))
             return false;
 
+        get_option(node, rhs.layout);
+
         return true;
     }
 };

src/config/config.h

@@ -135,6 +135,8 @@ struct package_diagram : public diagram {
     virtual ~package_diagram() = default;
 
     diagram_type type() const override;
+
+    option<layout_hints> layout{"layout"};
 };
 
 struct config : public inheritable_diagram_options {