add support for functions in production rules

lindenmayer-system/lindenmeyer-system.js

@@ -20,7 +20,13 @@ function generate(start, rules, iterations) {
         while(pos < start.length) {
             let w = start.slice(pos, pos + rule[0].length);
             if(startswith(w, rule[0])) {
-                start.splice(pos, rule[0].length, rule[1])
+                var ruleres;
+                if(typeof rule[1] == 'function') {
+                    ruleres = rule[1]();
+                } else {
+                    ruleres = rule[1];
+                }
+                start.splice(pos, rule[0].length, ruleres)
                 pos += rule[0].length;
             } else {
                 pos += 1;