Can't pass array to configure as first argument

I use `.configure` to get environment, and pass two different path. I think it should work that pass to `FileSystemLoader `, but it didn't.

```
nunjucks.configure(['views', 'widgets']);
```

Is something wrong with `isObject`, I think it should be

```
exports.isObject = function(obj) {
    return ObjProto.toString.call(obj) === '[object Object]'
};
```