Add background grammar rule

src/fart.propane

@@ -2,6 +2,7 @@ tree;
 
 drop /\s+/;
 
+token background;
 token comma /,/;
 token gt />/;
 token height;
@@ -15,20 +16,21 @@ token width;
 token uint /\d+/;
 token number /-?\d+(\.\d+)?/;
 
-Start -> Options? Scene;
+Start -> RenderParams Scene;
+
+Background -> background Vector;
 
 Height -> height uint;
 
 Number -> uint;
 Number -> number;
 
-Options -> options lcurly OptionsItems rcurly;
+RenderParams -> ;
+RenderParams -> RenderParam RenderParams;
 
-OptionsItems -> ;
-OptionsItems -> OptionsItem OptionsItems;
-
-OptionsItem -> Height;
-OptionsItem -> Width;
+RenderParam -> Background;
+RenderParam -> Height;
+RenderParam -> Width;
 
 Scene -> scene lcurly SceneItems rcurly;