[TwigBundle] Created stopwatch tag for profiling templates

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | symfony/symfony-docs#2630

This PR adds a new tag to Twig which you can use to time parts of a template and see it in the timing tab of the profiler.

Usage:

````jinja
{% stopwatch foo %}
... some things that gets timed
{% endstopwatch %}
````

This feature is based on the idea of @lsmith77 (see also our twitter conversation: https://twitter.com/lsmith/status/330772561413672962 )