Forward along components when passed as a prop

[johno]

MDX element creation dropped the components prop
when it was explicitly passed as a prop. For now
we will continue to forward it along.

For the MDX specific component prop, in the future,
we should consider potentially prefixing as mdxComponents.

Closes #635

[vercel]

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[ChristopherBiscardi]

This reduces weird behavior to this case:

<Component components={{h1: ...}}>
<h1> things</h1>
</Component>

and allows components to be passed through unmodified to the consuming component. I see this case coming up much less, so lgtm.