Add require relative load support

To use local AMD modules within extra.d plugins, the relative URL for
the global requireJS configuration makes this a bit difficult. To solve
this the following code can be added to plugin files, before the first
define call.

```
require.config({
	paths: {
		'extra.d': '../../../extra.d',
	}
})

define(['./local-module'], function(localModule) { /* ... */ });
```

This change adds this configuration by default, so it is no longer
required to have it in each plugin which want to use './local-module'
syntax in require or define. As all plugins loaded via `extra.d` folder
are inside `extra.d` this automatically means that the URLs are resolved
correctly from the extra.d base URL as defined by the plugin itself.

static/js/main.js

@@ -56,7 +56,9 @@ require.config({
 
 		'partials': '../partials',
 		'sounds': '../sounds',
-		'translation': '../translation'
+		'translation': '../translation',
+
+		'extra.d': '../../../extra.d'
 	},
 	shim: {
 		'modernizr': {