Accept remark and rehype plugin props to the markdown component

[PeteDarinzo]

Hoist P/R Checklist

Pull request authors: Review and check off the below. Items that do not apply can also be
checked off to indicate they have been considered. If unclear if a step is relevant, please leave
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• Caught up with develop branch as of last change.
• Added CHANGELOG entry.
• No breaking changes.
• Updated doc comments / prop-types.
• Mobile testing not required.
• Toolbox branch not required.

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