Ignore node_modules
Point reveal.js to origin's latest version
Update to markdown plugin's new API
Lock in reveal.js to stable version
Pass `opts` object to Markdown slidify
Rename vertical to verticalSeparator to match API

I personally like to match options with their full names, so
decided to carry 'verticalSeparator' all the way through from CLI to
MD plugin.
Merge pull request #6 from samtsai/revealjs_from_origin

Revealjs from origin