MacDown should be more relaxed with LineBreak or EmptyLine between markup sequences: Bodytext, list, heading

Nobody has claimed this yet.

## Sample
```
My list:
- Item one
- Item two should have this help text:
## Help text heading
Some prose here.
```

## Syntax Issue 1
`item one` requires empty line above to be recognized as bullet list. Already reported in #531

## Syntax Issue 2
`## Help text heading` requires an empty line above to be recognized as a headline.

## Side by side comparison of [QLMarkdown](https://github.com/sbarex/QLMarkdown) vs [MacDown](https://github.com/MacDownApp/macdown)

## Proposal
- Would be very desireable if the Mac QuickLook plugin QLMarkdown and the Mac Markdown WYSIWYG Editor MacDown would come to the same results.

## Considerations
- As far as I see it there's no problem with being more laissez fair with markdown interpretation:
- If less (linebreak only) leads to the same result as more (empty line)
- then adherents of both flavors could continue using their files and preferred markup and still get the same output (rendering).
- Of course in a collaborative environment (such as source code management) they will get collisions, but for individual use of popular Mac read-only and edit+preview apps they will get consistent results at least, no need to enforce one style only.

Start by reproducing the fenced sample in MacDown and compare its list and heading rendering with the linked QLMarkdown result. Done means the list item and help-text heading render correctly when separated only by a line break, without requiring an empty line.

Written by the indexing model from the issue text.