Add a blank line between viewport header and task list.

[jahagirdar]

Pandoc recognizes a list as a list only if it is preceded by a blank line. otherwise it treats it as a plain text and all the tasks get clobbered in a single line without any formatting.

[ajkavanagh]

Yeah, this is annoying me as well. And we're not the only ones, as PR #430 has be added to address this. Now it just needs to be merged. However, that would probably break other people's workflows, sadly. Thus it should probably be an option, which is why it's not been merged. Personally, I'm forking, doing the #430 change and seeing if any breakage occurs.