Add Markdown asset extension to core

[zzarcon]

As requested on #970 this adds Markdown support to parcel core instead of having to use a plugin for it.

Please let me know if Im missing something! 💃

Closes #970

[DeMoorJasper]

Imo this is a tricky one to implement, what if someone wants to import markdown and expects html?

Perhaps something like this would be better?
Markdown file:

exports.html = ''; // Html version of the markdown
exports.md = ''; // The original markdown string

Import usage:

const {md, html} = require('./somefile.md');

If someone only uses the html, treeshaking will remove the md one and vice versa

Another usecase might be linking to a markdown file from html, should this resolve to a html file or a markdown file?:

<a href="./somepage.md"></a>

[zzarcon]

Good point about the html version! I think we can easily have both named exports if we use a markdown converter like https://github.com/showdownjs/showdown. We might need to do the html conversion on demand to avoid unnecessary work.

About

<a href="./somepage.md"></a>

I'd say we want to provide the html version, that's what I will expect, but I'm happy to see other approaches :)

I think we need to decide first on, before continue:

• Which parser we want to use
• Whats the default behavior for md links

thoughts?

[DeMoorJasper]

@zzarcon Sounds good, we can always swap out the parser if it's not sufficient.
I think md links in html should transpile to html.

[zeakd]

Here's some markdown parsers

• markdown-it (used in stackedit.io)
• showdown
• marked (used in vue example)
• remarkable (used in react example) but it maybe not maintained.

I think md should be transpile to html, too. But the problem is how parcel can pass renderer to MarkdownAsset. Without renderer options, MarkdownAssets would not be useful. So,, which is better way?

• Pass renderer or other options to <link /> attribute
• external Markdown options like .markdownrc.
• else ?

[devongovett]

Going to close this. I think if we support markdown it should be more useful than just inlining the original string. For that, you can always use fs.readFileSync.

Feel free to open a PR that compiles markdown to HTML. Then maybe we can use something like #1528 to load it.