stops parallax failing if only the single horizontal slide and multiple vertical

check for 0 division, stops parallax failing if only the single horizontal slide and multiple vertical

Update the theme README.md

Fix text being cut off in Chrome when printing.

Fixed #1379 - config keyboard now disables 's' key

Fixes #1225

always enable highlight.js to fix markdown-only usage

Add more support for external markdown.

Use en dash in title; <meta> is a void element.

move up presentation size section

This brings together all configuration options.

The 'grunt watch' command now also watches for changes to external
markdown.
The 'grunt package' command now packages all external markdown files for
rapid deployment in static enviroments.