Pass MDAST markdown options

[glitteringkatie]

I was using remark-deflist with MDX and started to get some errors (namely that mdxSpanExpression didn't exist!). I saw in the docs for mdast-util-mdx-jsx that the key to fixing it was some options.

In sleuthing this out, I saw that there were two other issues that I think would be fixed by contributing this back!

I debated between just adding these mdx settings directly or passing them as options to remark-deflist. I decided upon utilizing options (and adding some docs on what options to pass in) for the sake of flexibility.

Quick note: I had prettier turned on and it got aggressive. Let me know if you want to try the changes again without the prettier changes but I think consistent syntax isn't bad 🤷. The main difference within packages/remark-deflist/src/index.js is the passing around of options.toMarkdownOptions and options.fromMarkdownOptions.

[glitteringkatie]

I was debating between using a unified pipeline or mdx itself and decided on mdx itself because the output ended up being more of what I expected (like jsx and correct html nodes as opposed to literal braces and unknown nodes becoming <div>s).

And then since I used mdx here, I kept it consistent with the example in the documentation.