Merge pull request #106 from jirutka/patch-16

Fix inline_footnote – handle unresolved xref
map open blocks to LaTeX environments

The converter now maps open blocks to LaTeX environments.
See the README, section 4.3, "LaTeX Environments" for details.
Briefly, a file, "new_environments.tex" is generated which
contains a simple but serviceable definition of a LaTeX
environment corresponding to the given open block.  These
definitions are stored in the file "new_environents.tex"
The user can use this file as a starting point for customized
environments.
map open blocks to LaTeX environments

The converter now maps open blocks to LaTeX environments.
See the README, section 4.3, "LaTeX Environments" for details.
Briefly, a file, "new_environments.tex" is generated which
contains a simple but serviceable definition of a LaTeX
environment corresponding to the given open block.  These
definitions are stored in the file "new_environents.tex"
The user can use this file as a starting point for customized
environments.
use .adoc file extension
migrate latex backend to asciidoctor/asciidoctor-latex repo

- see https://github.com/asciidoctor/asciidoctor-latex
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 doctest to 1.5.0
Update doctest to 1.5.0 (rename examples)
Add missing block_outline template to HTML5/Haml