Update commonmark spec from upstream (and reflect its changes)

[mity]

This PR uses (yet unreleased) Commonmark specification from the spec repo and implements changes mandated by it.

Quite possibly won't be merged into trunk until the next spec version gets released.

Update: spec 0.31 has been released.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (5bd6224) 91.75% compared to head (584d389) 91.76%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #208 +/- ## ========================================== + Coverage 91.75% 91.76% +0.01% ========================================== Files 5 5 Lines 3410 3402 -8 ========================================== - Hits 3129 3122 -7 + Misses 281 280 -1 ```

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[mity]

cc @ec1oud

Spec 0.31 has just been released. This changes my plans: Will make release as soon as I get some confidence the quality (with this PR) is no worse than 0.5.1.

The respective changes in code (other than update of the punctuation table) very local and quite benign so I don't expect troubles.