cameo: allow comma between parameters of "drill" (makes use in scripts easier)

- lang.y: allow an optional comma between the parameters of "drill"
- README: explain the new syntax of "drill"

cameo/lang.y

@@ -310,7 +310,7 @@ command:
 			gnuplot_append($2, paths);
 			translate(paths, 0, 0, -zo);
 		}
-	| TOK_DRILL dimen dimen
+	| TOK_DRILL dimen opt_comma dimen
 		{
 			struct path **walk;
 
@@ -319,7 +319,7 @@ command:
 			walk = &remain;
 			while (*walk)
 				walk =
-				    classify(walk, try_drill(*walk, $2, $3));
+				    classify(walk, try_drill(*walk, $2, $4));
 		}
 	| TOK_MILL dimen dimen
 		{
@@ -433,3 +433,7 @@ offset_option:
 			$$ = OO_INSIDE;
 		}
 	;
+
+opt_comma:
+	| ','
+	;