New option allow_zero_leading_decimals.

This PR allows decimal string to be started with multiple 0s if the related option is given.

I don't know if this change comply with JSON RFC but it seems that leading zeros in decimal number seems to be valid in js:

```js
 > 012389
=> 12389
 > 000012389
=> 12389
```

Similar approach documented at https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/parseInt.