[HOTFIX] Accept html5 tags

Related issue: https://github.com/sulu-cmf/SuluContentBundle/issues/163

Preview render throw an exception for html5 tages (e.g. <nav>):

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | no => WebsiteBundle Hotfix https://github.com/sulu-cmf/SuluWebsiteBundle/pull/29
| Fixed tickets | https://github.com/sulu-cmf/SuluContentBundle/issues/163
| BC Breaks     | no
| Doc           | none