Support “timeline” definition lists

These make it easier to construct timelines by just doing

    {.timeline}
    : 1972-01-01

      - Here is a thing which happened on Jan 01 1972

    : 1972-12-31

      - Here is a thing which happened on Dec 31 1972

There’s a lot of work which could be done to style these, but it’s at
least semantically appropriate.