Previously, all Markdown blocks had a virtual parse method which has
been swapped out for a static parse method returning an OwnPtr of
that block's type.
The Text class also now has a static parse method that will return an
Optional<Text>.
You can now continue one list item across several lines, as long as you indent
it the same as the first line of the item. For example:
* This is the first item,
which spans multiple lines.
* This is the second item, which also has some
*emphasis* in it.
