Merge pull request #45 from aheusingfeld/master

Added 'revealjs_customtheme' attribute to specify custom theme files
add details div around document meta
Update block for new FontAwesome syntax

Update block for new FontAwesome syntax. In asciidoctor 1.5.0 font awesome has been updated to latest version. In this new version fontAwesome changed the way how icons should be set. This commit updates this backend with the new names.
embed dzslides resources unless link-assets is set
mention link-assets attribute in README for dzslides backend
fixed link to print stylesheets