[DomCrawler] Added node name getter

[DomCrawler] Added node name getter

Based on the use case came up in a SO question (http://stackoverflow.com/questions/22564465/getting-the-tag-name-of-a-element-crawled-with-domcrawler-in-php) I propose a new helper function to get the name of the active node

| 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#4039

(Follow up of symfony/symfony#11351)