There have been some discussions regarding Jekyll style front matter blocks, along with other approaches. See the topics listed under “Meta Data” in the list of proposed extensions.
Front matter is not going to be part of the spec. An application that uses CommonMark can employ its own conventions for extracting lines from the front of the document and parsing them as front matter. In general, the needs will be different for different applications.
If you want something that works out of the box, try my https://github.com/jgm/lcmark, which allows YAML front matter and has a templating system.