Update defaults to match CommonMark & Github conventions

[zm-cttae]

Closes #424.
This pull introduces the CommonMark (now Github flavor, Reddit flavor) conventions that people use.

CommonMark is so popular compared to Josh Gruber's original Markdown spec that we should favor it.

[zm-cttae]

@pavelhoral please review!

[domchristie]

Are these defaults specified anywhere? (FWIW the current defaults originate from the John Gruber’s original markdown spec)

[zm-cttae]

John Gruber's spec - the first set of conventions - aren't really used as much nowadays 🤔

These GitHub conventions are all listed here, but never explained as "conventions":

• https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/quickstart-for-writing-on-github
• https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax

[zm-cttae]

Good news 🎉 - CommonMark's Markdown reference help page clearly matches and favours these conventions Per https://commonmark.org/help/

[acnebs]

There is a type in the commit, codeBlockStyle should be fenced not fence.

[zm-cttae]

Cheers thanks for that @acnebs

[huan086]

br shouldn't be change from hard break '␠␠' to soft break '\n'. When converting to HTML, Common mark states that conversion of hard break is to br, but soft break stays as '\n'. In HTML, '\n' is shown as a space unless the element is style with the various pre styles

[zm-cttae]

✅ Done @huan086

[za3k]

You should get the tests to pass.