Fix inline_footnote - handle unresolved xref

To be consistent with the built-in version of the backend.
Update doctest to 1.0.0.rc.3
Refactor slim/html5 templates
Add slim/html5/block_outline template
Fix html5/block_table - use stylesheet to set default table width

Reflects https://github.com/asciidoctor/asciidoctor/commit/d2537aacb6834bedf58ff6f5db4bd88afd67ce79
Add support for YouTube playlists to html5/block_video
Add support for video start/end anchor to html5/block_video
Update examples/html5/inline_quoted