Implemented multi-block mode.

Many blocks will only have a single child.  When this is the case
it is a waste of space to have a single line for the parent.

With this change, it is possible to transform

    |list
        |item
            |math a + b = c
        |item
            |math a^2 + b^2 = c^2

into

    |list
        |item/math a + b = c
        |item/math a^2 + b^2 = c^2

Much better.