Add dot notation support

According to the [Mustache spec](https://github.com/mustache/spec), the dot notation should be available. 

```
Hello {{person.name}}
```

Other Mustache implements such as [mustache.js](https://github.com/janl/mustache.js) have already done the job.

I've achieved the feature in the `lookup` function, and added some [tests from the spec](https://github.com/mustache/spec/blob/master/specs/interpolation.yml#L124)