youtube/vimeo/video tag improvements

- Allow more flexible video identifiers, current one "\w" blocks e.g.
youtube identifier "ul-FqOfX-t8". New one allows also attaching
positional arguments to video, e.g. "ul-FqOfX-t8#t=123"
- Surrounded each video element with a div having "videobox"-class for
styling purposes (e.g. center alignment)
- improved code formatting
Merge pull request #207 from jojanaho/master

Liquid tags: youtube / vimeo / video improvements