Add `format` to support MDX and plain markdown

[wooorm]

Add format to support MDX and plain markdown

• Default to detecting whether input is mdx or md based on whether the given file has an extension in mdExtensions and to handle both mdExtensions and mdxExtensions in integrations
• Support format: 'md' to treat input as markdown and to handle mdExtensions in integrations
• Support format: 'mdx' to treat input as MDX and to handle mdxExtensions in integrations
• Switch which elements will turn into configurable components, previously that was everything that was written in the source markdown, now this is everything not explicitly written as JSX
• Add tests for integration with rehype-raw
• Fix docs on evaluate options

Closes GH-17.

[wooorm]

@ChristianMurphy Do you see the default being detect as a good idea? I think so

[wooorm]

/cc @remcohaszing @ggoodman WDYT?

[ChristianMurphy]

Do you see the default being detect as a good idea?

I think so. :+1: There is an edge case, that being if people use non-standard md and mdx extensions and still want the type to be inferred there isn't a way to override at the moment. But until that use case pops up, YAGNI

[wooorm]

There is an edge case, that being if people use non-standard md and mdx extensions and still want the type to be inferred there isn't a way to override at the moment.

The way I’ve set it up now is that there’s two lists of extensions: one for markdown, one for mdx. Folks can overwrite those to set their own funky preferences.

It’s indeed a weird case, but with {mdExtensions: ['.mdx', ...], mdxExtensions: [...]} one could load .mdx as markdown, or with {mdExtensions: [...], mdxExtensions: ['.md', ...]} one could set .md as MDX. Again, I think it’s really weird, but it’s possible.

[remcohaszing]

I mostly looked at the docs, because although I’ve looked at the internals, I’m not very familiar with them.

Anyway, looks good to me :+1: