Add support for Markdown in HTML pages

What

Add a Markdown renderer to the kit dependencies.

Configure the prototype kit server to render all page HTML in the user’s prototype with the Markdown renderer after it has been rendered with Nunjucks.

Configure the Markdown renderer so that by default the output HTML has GOV.UK Frontend classes, but leave the option open of the user changing this.

Why

See #46 for details. This has been a hotly requested feature for ages.

Who needs to work on this

Developer

Who needs to review this

Developer, Designer, Tech Lead

Done when

• Prototype authors can opt-in to using Markdown in their prototype
• Prototype authors can use Markdown syntax in any part of any .html file in app/views
• HTML rendered by Markdown has GOV.UK Frontend styles by default
• Users can opt-out of GOV.UK Frontend styles for Markdown if they wish

[nataliecarey]

To clarify - settings right now can be changed in config.json, that’s good enough for this ticket. I’ll create another ticket for the settings page. At the moment I’m thinking it would be good to release them together if we can but this piece of work doesn’t rely on the settings page, just the setting in config.json.

[joelanman]

Thanks! It’s not a strong opinion but I’m just wondering if one benefit is making it easier for our users to understand the code they’re seeing. A tag would say explicitly ‘this is markdown’, in the same way curly braces say ‘this is nunjucks’