Adding support for custom markers

[d4rekanguok]

Hi Neal, thanks for the awesome plugin!

I wanted to use the plugin using a different syntax other than code block ``, since it prevents netlifyCMS to correctly parse & display a preview, described in this stackoverflow question. It might also helps with #13 if I understand that one correctly.

I have been writing a custom tokenizer plugin for remark, aiming to make it easier to write custom static component for markdown in gatsby. I implemented the feature using my plugin here.

With this, it allows user to use self-defined syntax, such as

[[youtube: gdviJzEwShE]]

or

---youtube: gdviJzEwShE---

It still support the inline code syntax `youtube: gdviJzEwShE`, and will not invoke the custom parser unless user specifies a custom syntax.

edit: Here's a test gatsby site

Within Gatsby's eco system, using inline code will cause no issues, so I understand this is a niche usage. If you like what I've done, I would love to send over a PR & discuss further. If not, I'd like to ask for your permission to publish my fork as a separate package on npm.

Thanks again!

[borgfriend]

Sorry, I am currently busy with other things... but a very cool idea, that was one of the things I wanted to actually have from the beginning - but well - time constraints.

If you send over a PR then Ill take a closer look at it and merge it in.

[Joel-Mercier]

This would be very helpful since I can't have both video embeds and code highlighting at the same time at the moment because of the syntax used by this plugin