Add formatter for MDX files

[anmonteiro]

https://mdxjs.com (Markdown + JSX)

[sbdchd]

Looks good!

Could you also update ‘docs/neoformat.txt’ and the ’README.md’?

Thanks

[anmonteiro]

Thanks for the review. I'm happy to do that but I couldn't get this working yet because the filetype I get is actually markdown.mdx (compounded filetype).

As per the documentation:

When a dot appears in the value then this separates two filetype
names. Example:
/* vim: set filetype=c.doxygen : */ ~

Does neoformat support compound filetypes like the above?

[sbdchd]

Yeah I don’t think it does. I think the only solution currently is to set the file type to just be ‘mdx’