Allow requiring ".handlebars" files within Node.js scripts

When using handlebars for server-side templates and storing those templates in individual files, it is convenient to be able to load and compile those files into template functions using `require()`, like so:

```javascript
var handlebars = require("handlebars");
var template = require("./template.hbs");
var text = template({name: "Jesse Clark"});
```

This patch provides a `require()` handler that loads and compiles `.handlebars` and `.hbs` files.