Merge #46:
- Require PHP 5.5 or 7.0 and Symfony 3.4 minimum
- Refactored extension and enabled autowiring
- [BC Break] Removed classes parameters
- [BC Break] Removed the form data transformer
- added a new text form type extension with a purifier listener to purify submitted data in all text based fields, using opt-in and custom profile thanks to dedicated options
- added a new "exercise.html_purifier" tag to make custom purifier implementations available as profile through form options and Twig filter
- added a purifiers registry to lazy load purifiers everywhere
- added a Twig HTMLPurifierRuntime for better performances
- upgraded the LICENSE and README files