set options when converting to HTML

* force the backend to html5 (so document cannot override)
* set safe mode to allow font-based icons and other features to be enabled
* show the document title by default
* enable font-based icons by default (but allow document to disable, hence the trailing `@`)
* enable source-highlighter to highlight.js (since it's used automatically)
* set the platform / env attributes to allow for conditional rendering
* update $render method to $convert