Include partials from `node_modules`

This PR adds the site's `node_modules` folder as a [partials dir in express-handlebars](https://github.com/ericf/express-handlebars#partialsdirviewspartials). All modules installed by the site can then be used for partials:

```
{{> some-package/views/news}}
```

Note that those views will always be cached, even in dev.

Fixes #104.
Fixes #123.