match remark's filter option

[ChristopherBiscardi]

In gatsby-mdx we have (a private, undocumented API that you shouldn't use) shouldBlockNodeFromTransformation. gatsby-transformer added a filter function to achieve similar functionality at some point.

TODO:

• figure out if these were added so that gatsby-remark-transformer can have multiple instances or not.
• figure out what the rest of the story is with the options and if we should adopt them.

[ChristopherBiscardi]

This got reverted in remark. gatsbyjs/gatsby#12639