Add Option to Know Which Files are Processed As Markdown

This allows users of the plugin to know which files started as markdown and which files didn't.

My usecase for this is in a template that applies a special class to content that started as markdown to give it some basic styling that markdown expects to be in place. This same styling I don't want to be applied to cotents that started as html as those pages are generally more advanced and don't need to basic markdown styling.