list items only contain content that is indented to their respective
level. Except when that content is inside a block. To allow for this we have to
ignore the parentStop when parsing a block and just include everything until
the end of that block.
Can't think of any problems with this right now. Let's see if this comes
back to bite me.
Until now we expected the .org file to print back to itself - we can't do that
when the input is not pretty printed already - with the introduction of blocks
with unindented content that will be the case.