fix(position): add CSS for webpack support

Closes #5561

Author: Foxandxss

src/position/index.js

@@ -1,5 +1,7 @@
 require('./position');
 
+require('./position.css');
+
 var MODULE_NAME = 'ui.bootstrap.module.position';
 
 angular.module(MODULE_NAME, ['ui.bootstrap.position']);