Add subsessions for agenda shortcode

It is a common request to have sub sessions. We have been displaying sub-sessions in abstracts or as their own sessions titled as main session: sub-session name.

The long term solution should be to just add an indent level field in agenda.yml. This way we can visually emphasize that the session is a sub-session without breaking out of the table format, or having verbose and repetitive titles.

items:
  - name: Some session
  - name: Some sub-session
    level: 2  
  - name: Some other session