Allow customizing a template extension in requirejs config

Let the developer override the template extension in the requirejs config file.
Otherwise it defaults to '.hbs' template extension. 
Added this becouse it didn't require too much overhead, while still letting me get syntax highlighting sugar from my editor :/ 

```javascript

require.config({
    paths: { ... },
    hbs: {
         templateExtension: '.html'
    },    
    shim : { ... },
});

```