…ame-only tags (wouterj)

This PR was merged into the 3.3-dev branch.

Discussion
----------

[DependencyInjection] Added Yaml syntax shortcut for name-only tags

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | todo

This PR adds a little shorcut for tags without any attributes. There are increasingly more name-only tags in Symfony and having to do `{ name: twig.extension }` for these seems way too verbose to me.

**Before**
```yaml
services:
    app.twig_extension:
        class: AppBundle\Twig\AppExtension
        tags:
            - { name: twig.extension }
```

**After**
```yaml
services:
    app.twig_extension:
        class: AppBundle\Twig\AppExtension
        tags: [twig.extension]
        # or
        #    - twig.extension
```

This of course means we introduce a new format to achieve the same goal. I believe this isn't a big problem as the decision is distinctive and simple: If you configure tag attributes, use the long format, otherwise use the short format.

Backwards compatibility
---

In this PR, an exception was removed to allow this new shortcut format. The BC promise doesn't cover exceptions and I think removing the exception here should cause anything to break:

 * Applications shouldn't rely on exceptions
 * If code was triggering this exception before, it would not cause any behaviour change after this PR: The service just retrieves an unused tag, which is simply ignored by the container.

Commits
-------

7fa8c8a Added Yaml syntax shortcut for name-only tags