ANN: CommonMark 0.20, cmark 0.20.0, commonmark.js 0.20.0

Although there is lots of serious work to be done on the spec (e.g. on HTML blocks), I thought it would be useful to have an incremental update.

Thanks to everyone who gave feedback, reported typos or bugs, or submitted pull requests. Thanks especially to Ben Trask and Nick Wellnhofer for their work on making cmark more robust.