Add external link markup

[michaeltlombardi]

As an author, I want to be able to mark external links that open in new tabs accessibly.

Proposed Markup

Input Markdown

[external:Deploy to Netlify](https:netlify.app)

Output HTML

<a href="https://netlify.app" target="_blank" rel="noopener noreferrer">
  Deploy to Netlify
</a>

Rendered Output

Deploy to Netlify

Options

Because this uses link markup, instead of specifying the style as an attribute or title (because the title is a valid link component), instead use the prefix:

• external and external-default map to data/_params/platen/links/external/_default.yaml

• external-foo maps to data/_params/platen/links/external/foo.yaml

• The prefix bar defined as an alias:

  platen:
    markup:
      links:
        aliases:
          prefix: bar
          definition: my_theme.bar

  Maps to data/_params/platen/links/my_theme/bar.yaml

Users should be able to set at least the following attributes:

• target
• rel
• href
• class

Where href is overridden by the value specified for the link itself.