Exporting custom path in md component does not work

[jurajhrib]

Hi, First of all, thank you for your awesome work on all the libs!

I noticed the export const path = '...' doesn't seem to work for me. I checked the code and by the looks of it, you only add custom exports when it's the Index component. And the exports are loaded in promise, so the path would not get there anyway I guess.

https://github.com/jxnblk/mdx-go/blob/master/lib/client/routes.js#L58

I am trying to achieve custom and nested links with custom menu, so that's why I needed this. I really like mdx-go and was avoiding using Gatsby, because this was more than sufficient for my needs.