Fix custom blocks not using custom classes

[giraffesyo]

Fixes #6366

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[m-allanson]

This looks good, thanks @giraffesyo.

What do you think about changing the config format of gatsby-remark-custom-blocks to match the format expected by remark-custom-blocks? Then we can just pass the options straight through, and it won't be a breaking change if this feature wasn't working before.

[giraffesyo]

@m-allanson If you pass the options as documented on the remark-custom-blocks repo it works as expected, as I noted in #6366. So, in that case it would just be a documentation change. My question is, was there originally a reason to have a different format from remark-custom-blocks?

[m-allanson]

👍 Yeah let's do this as a documentation change.

My question is, was there originally a reason to have a different format from remark-custom-blocks?

I don't know for sure, but I can't think of a good reason why having a different format would be useful here.

[m-allanson]

Now that's some nice docs. Thanks :)

I made one minor comment but otherwise this looks great 🎉

[m-allanson]

delete?

[giraffesyo]

Whoops, had a train of thought there that went astray. I added a commit to fix this. Let me know if there should be any changes.

[giraffesyo]

Thanks @m-allanson!

[m-allanson]

Nice work, thanks @giraffesyo 👍