Update GFM support

[mlopezgva]

Hi!

Would it be possible to add the most recent additions from github and other extras to the project?
Some nicities I would like to have:

• checklists (like this list :P), like gitHub or Markable.in
• ==text hightlighting== (inline bright yellow background, like Haroopad)
• support for PlantUML diagrams, either local or remote (see PlantText for an online implementation)

Thank you for you time and these programs.

[philippe-git]

Hey @mlopezgva!

Thanks for those ideas! I think they sound great and reasonable, and overall shouldn't slow down the parsing and previewing.

On the other hand, MME uses https://github.com/markdown-it/markdown-it for parsing Markdown, which itself aims to follow the CommonMark spec. This is a spec whose objective is to bring all Markdown tools and services closer together, by deciding which features Markdown should support as usage evolves. Not all Markdown features out there are part of this spec, and as such, not all Markdown features out there are supported by the Markdown parser MME uses.

From this list, I think none of those features are part of the last version of the CommonMark spec. Fortunately, the Markdown parser is super extensible, so it does allow some additional features to be implemented outside of that spec! I've only seen such an extension for (2) text highlighting though, so that's the only one we'll be able to bring to MME for now.

I'm not sure about PlantUML diagrams, but I have a hunch checklists are getting pretty popular, so I think we may be able to add support for them in the future too :)

Thanks again for those suggestions – I'm currently on and off working on a huge React + Flux refactoring of the app to merge the web + Chrome apps into a single codebase and making it faster to maintain and implement new features, and I'd love to make that addition once this big project is done :) I'll leave that issue open and close it once text highlighting is there!