resolves #1105 support custom YouTube playlists

- build custom playlist from comma-separated target on video macro
- add test for custom playlist
- fix fullscreen settings for HTML5 player
- add modest option to enable modestbranding param
- add theme attribute to control theme param
- add hl param if lang attribute is set on macro or document
- add relative option to control rel param
- don't bubble start and end attributes
- cleanup some logic in the video convert method
- remove trailing whitespace from html5 converter file