Add Node.js require() handler for .dust files

When working on server code with templates in individual `.dust` files, it is convenient to be able to `require()` those files into useful modules, like so:

```javascript
var dust = require("dustjs-linkedin");
var helloTemplate = require("./templates/hello");
helloTemplate({name: "Nemo"}, function(err, result) {
    if (err) throw err;
    console.log(result);
});
```

This patch provides a `require()` handler to make that possible.