Kewords extractor

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #117 
| License       | MIT
| Doc PR        | add extractor to docs https://github.com/symfony-cmf/symfony-cmf-docs/pull/438

This PR adds an extractor for keywords.