Add line endings to HTML blocks

[chenzhiguang]

1. If an HTML block has a parent block, add a leading line ending
2. If an HTML block is a child of a list, add a trailing line ending.

[coveralls]

Pull Request Test Coverage Report for Build 4068382883

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 96.047%

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Totals
Change from base Build 4068178952:	0.005%
Covered Lines:	1385
Relevant Lines:	1442

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💛 - Coveralls

[chenzhiguang]

@srawlins Hi Sam, this is a simple one.