fix multi-line section titles

[yshavit]

If I have markdown:

This is a
multi-line section title
---

Hello, world

then that gets rendered as a multi-line section heading:

This is a
multi-line section title

Hello, world

mdq doesn't handle this correctly when it renders.

Note that it only seems to work with the --- or === syntax. With #, each line is its own section, with the first being empty:

# This is a
# multi-line section title

Hello, world

This is a

multi-line section title

Hello, world

Discovered while testing #369