[4.2] Router : add subdomain feature

Simple yet useful (IMO) : add `subdomain` feature to `Router`.
Of course, if no `domain` is set, `subdomain` becomes useless and is not taken into consideration.

The following:
````php
Route::group(['subdomain' => 'test'], function () {
	// http://*/test
	Route::get('test', function () {
		return 'no domain';
	});
});

Route::group(['domain' => 'bar.baz'], function () {
	// http://bar.baz/
	Route::get(null, function () {
		return 'no subdomain';
	});

	// http://api.bar.baz/
	Route::get(null, ['subdomain' => 'api', 'uses' => function () {
		return 'api subdomain for bar.baz';
	}]);
});

Route::group(['domain' => 'foo.com', 'subdomain' => 'www'], function () {
	// http://api.foo.com/
	Route::get(null, ['subdomain' => 'api', 'uses' => function () {
		return 'api subdomain forced';
	}]);

	// http://baz.test/
	Route::get('override', ['domain' => 'baz.test', function () {
		return 'group domain overridden : www subdomain is not taken';
	}]);

	// http://www.foo.com/
	Route::get(null, function () {
		return 'get group domain and subdomain';
	});
});
````
will provide these routes:
```sh
+-------------+-------------------+------+---------+----------------+---------------+
| Domain      | URI               | Name | Action  | Before Filters | After Filters |
+-------------+-------------------+------+---------+----------------+---------------+
|             | GET|HEAD test     |      | Closure |                |               |
| bar.baz     | GET|HEAD /        |      | Closure |                |               |
| api.bar.baz | GET|HEAD /        |      | Closure |                |               |
| api.foo.com | GET|HEAD /        |      | Closure |                |               |
| baz.test    | GET|HEAD override |      | Closure |                |               |
| www.foo.com | GET|HEAD /        |      | Closure |                |               |
+-------------+-------------------+------+---------+----------------+---------------+
```
Maybe this would be better for 4.2 then.