Remark plugin does not prefix reference links

[i-like-robots]

Description

Links in Markdown files using the reference format are not currently rendered with the specified path prefix.

Steps to reproduce

For a project using markdown files as a data source and the gatsby-transformer-remark plugin, author a link in a markdown file using the reference style, e.g.:

This is [a link](/path/to/page).

This is [a reference]

[a reference]: /path/to/page

Build the project using the --prefix-paths option.

Expected result

The second link should be rendered with the path prefix, e.g.:

<p>This is <a href="/prefix/path/to/page">a link</a></p>

<p>This is <a href="/prefix/path/to/page">a reference</a></p>

Actual result

The first link will be rendered with the prefix and the second will not:

<p>This is <a href="/prefix/path/to/page">a link</a></p>

<p>This is <a href="/path/to/page">a reference</a></p>

Environment

  npmPackages:
    gatsby: next => 2.0.0-rc.25
    gatsby-remark-autolink-headers: ^1.4.19 => 1.4.19
    gatsby-remark-external-links: 0.0.4 => 0.0.4
    gatsby-remark-prismjs: ^2.0.5 => 2.0.5
    gatsby-source-filesystem: next => 2.0.1-rc.6
    gatsby-transformer-remark: next => 2.1.1-rc.5
    gatsby-transformer-yaml: ^1.5.18 => 1.5.18

[i-like-robots]

I've looked into this and I think this is because plugin is currently only expecting link nodes and not definition nodes:

gatsby/packages/gatsby-transformer-remark/src/extend-node-type.js

Line 141 in bab08df

visit(markdownAST, `link`, node => {

The solution may be to listen for both (unist supports providing an array of tests), I hacked this into my local install:

visit(markdownAST, ['link', 'definition'], node => {
	if (node.url && node.url.startsWith(`/`) && !node.url.startsWith(`//`)) {
		node.url = withPathPrefix(node.url, pathPrefix);
	}
});

Shall I create PR for this or is the above enough information for somebody better positioned or currently working in this area?