Let 'javascript' handle aggregation!

[jf-tech]

The current way of doing sum/avg feels awkward and clumsy. Since we have 'javascript' custom_func, let it handle aggregation. The idea is to have its argument decl support array:

from:

                "total_price": { "custom_func": {
                    "name": "javascript",
                    "args": [
                        { "const": "num * price" },
                        { "const": "num:int" }, { "xpath": "number_purchased" },
                        { "const": "price:float" }, { "xpath": "item_price" }
                    ]
                }}

to:

                "total_price": { "custom_func": {
                    "name": "javascript",
                    "args": [
                        { "const": "var total=0; for (i=0; i<Math.min(num.length,price.length); i++) { total+=num[i]*price[i]; } total; " },
                        { "const": "num:int:array" }, { "xpath": "number_purchased" },
                        { "const": "price:float:array" }, { "xpath": "item_price" }
                    ]
                }}