If the metadata field is all on one line, we try to interpret
it as Inlines, and only try parsing as Blocks if that fails.
If it extends over one line (including possibly the `|` or
`>` character signaling an indented block), then we parse as
Blocks.
This was motivated by some German users finding that
date: '22. Juin 2017'
got parsed as an ordered list.
Closes#3755.
