Use CommonMark instead of loosely defined Markdown flavour

[dumblob]

It seems that the precise specification of CommonMark already settled down, so I'm proposing sticking with it to be compatible with most of Markdown flavours and also be predictable.

CommonMark is backwards compatible with the original Gruber's Markdown, so it should be just internal change.

[nikolas]

I agree - it would be great if marked supported CommonMark! Currently I'm using Remarkable because it has a commonmark option.

[puzrin]

marked's strong point is it's small size.

For commonmark parser you'd better search in "official" list of implementations: https://github.com/jgm/CommonMark/wiki/List-of-CommonMark-Implementations. If you need commonmark support, i see no problem to select appropriate project.

[noraj]

It seems a good idea to use commonmark.js, markdown-it or remarkable.

[howardroark]

I really enjoy how flexible this parser is. Allows for a good amount of human error. Which is what you want from a syntax designed for writers.

[joshbruce]

See #956