Support new Slate schema

[pollonium]

Adding a plugin leads to the following error:

Uncaught Error: Schemas in Slate have changed! They are no longer accept a `rules` property.

[mxstbr]

Oh sorry, I forgot to update this repo! We've switched to DraftJS, so while this works ~relatively well it's no longer actively maintained. (reference: https://github.com/withspectrum/slate-mentions/issues/14)

Happy to move this repo to a new home if you want to maintain it!

[chilijung]

Hi @mxstbr, I've built a complete version of slate-markdown https://github.com/Canner/slate-md-editor, it would be great if you could mention us as the alternative in this repo, thanks!

[chilijung]

And also #2 github like codeblocks is also supported in our slate-markdown version!