get rid of "continue" when encounter objectid (issue #12)

I have a `user` property in one of my schema, like
``` javascript
user: {
        type: Schema.Types.ObjectId,
        ref: 'User',
        es_indexed: true,
        es_index: 'not_analyzed'
    }
```
The original code will skip both `es_indexed` and `es_index` options, so I commented `continue` when encountered `objectid` so that we can simply take `objectid` as a simple string while mapping those `es_` options as well.