EPUB writer:  Allow 'stylesheet' in metadata.

The value is a path to the stylesheet.
Add booktabs package for LaTeX tables

[ci skip]
Merge pull request #1076 from jlduran/booktabs

Add booktabs package for LaTeX tables
Use latest pandoc-templates.
Stop parsing "list lines" when we hit a block tag.

This fixes exponential slowdown in certain input, e.g.
a series of lists followed by `</div>`.
Allow https: to work in pandoc command line arguments.

(Use openURL from Shared instead of simpleHTTP.)