Support for importing sketches as namespaces

As a continuation on #5 

We should be able to call different functions(?) like ```sketch.root```or ```sketch.article``` on imported sketches. A made up example:

```
%import -myFakeArticlesSketchURL as gazette

root:
- <html><body><h1>Fake Gazette</h1>{{ gazette.article * 10 }}</body></html>
```