[Skeleton][Bundle] Add valid html via extends base to default template

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | no ticket found
| License       | MIT
| Doc PR        | n/a

Currently, the skeleton for the Bundle Generator produces an index.html.twig file that does not include standard HTML tags. Symfony2 profiler/toolbar requires standard tags (i.e. </body>) to display the need elements. In other words, the default controller and view created by the Bundle Generator will not show the profiler/toolbar when making a request via app_dev.php.

By adding the extends '::base.html.twig' and the proper title and body twig elements, the default controller and template produced by the Bundle Generator displays the proper profiler/toolbar elements when using app_dev.php. The result is a valid html page all other things being equal.