Workaround for github-flavored markdown not ignoring newlines within paragraphs

[krobelus]

I use sphinx-markdown-builder to generate release notes for GitHub
releases.

Unlike CommonMark, GitHub's flavor of Markdown interprets input like
line1\nline2 as line1<br/>line2.

As a workaround, I could write each paragraph in my *.rst sources in
only one line *.rst. But I'm not sure I want to do that; it would
be surprising to people who work on the *.rst sources because all
other builders (HTML, man, markdown+commonmark) handle this just fine.

Add a workaround to produce a faithful version GitHub-flavored
markdown by joining all multiline paragraphs into a single line.

I'm not aware of significant downsides yet (except that the Markdown
is uglier) but I haven't tested beyond my real-world use.

To be safe, I put it behind a config option, if only to not muddy
the waters as to which dialect of Markdown we are targeting.

Might as well make that option open for extensions, because there
are quite a few flavors:

$ pandoc --list-input-formats | grep mark -i | paste -sd ,
commonmark,commonmark_x,markdown,markdown_github,markdown_mmd,markdown_phpextra,markdown_strict

[coveralls]

Pull Request Test Coverage Report for Build 18552270670

Details

• 3 of 3 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.006%) to 98.344%

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Totals
Change from base Build 18529202121:	0.006%
Covered Lines:	772
Relevant Lines:	785

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💛 - Coveralls

[liran-funaro]

LGTM

[krobelus]

Thanks! I'd appreciate a release so I can use the PyPI package instead of my fork. But no rush.