Preserve whitespace more faithfully when processing %%% remarkup blocks

Summary:
This improves the output for some edge cases with `%%%` blocks. Notably, blocks did not emit a block-level spacing tag, so this:

```
%%%a%%%
%%%b%%%
```

...rendered on a single line.

Test Plan:
Added unit tests.

{F249620}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: chasemp, epriestley

Differential Revision: https://secure.phabricator.com/D10984