Markdown handling in content parse

[jasonrhodes]

By default, the content is just passed through as-is. Should we parse as markdown by default or should we make that a user decision, requiring a user to define a contentParse function that returns the parsed markdown?

Related: it would be nice to be able to parse content differently based on the file extension but right now Pagemaki.parse accepts the string contents of the file only. Need to think how to fix (maybe a separate issue).