[Whoops] Share whoops.handler instances, allowing easier handler customization

Currently Whoops handlers are not shared through the IoC container, so in order to customize pre-set Whoops handlers, such as the handler responsible for displaying the error page, one must get the `Whoops\Run` instance, retrieve all the handlers, and find the handler we want.

This PR shares the `whoops.handler` service by default, making it easier to perform tasks on the default Whoops handler:

```php
($app["whoops.handler"] instanceof Whoops\Handler\PrettyPageHandler)
  and $app["whoops.handler"]->setPageTitle("We get signal :(");
```