Added custom remark transform for Codepen example links

[bvaughn]

Relates to #246

Given a directory structure with Codepen examples like so:

Markdown links are automatically converted to HTML links to open the example code in question:

<!-- before -->
[](codepen://components-and-props/rendering-a-component)

<!-- after -->
<a href="/codepen/components-and-props/rendering-a-component" target="_blank">
  Try it on CodePen
</a>

<!-- before -->
[Try this Hello World example code on CodePen](codepen://hello-world)

<!-- after -->
<a href="/codepen/hello-world" target="_blank">
  Try this Hello World example code on CodePen
</a>

The generated links point to Gatsby redirect pages that use Codepen's prefill API to create a working pen with the linked example code.

Links are also verified to ensure that they reference valid example files. For example, if there is a link to codepen://components-and-props/rendering-a-component, our Gatsby plug-in will verify that the file /codepen/components-and-props/rendering-a-component.js exists. This will avoid broken links at runtime.

[reactjs-bot]

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