Make a Myriad plug-in for Thoth.Json

[travis-leith]

I think it would be awesome to have a Myriad plug-in that generates coders and decoders.
I suspect the resulting coders would be more efficient that the Auto ones, and hopefully much of the code that makes the Auto coders work would be useful in creating the plug-in.

https://moiraesoftware.github.io/myriad//tutorials/external-plugins.html