Liquid tags: youtube / vimeo / video 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