Support for MJML

[tarleb]

The website describes MJML as follows

MJML is a markup language designed to reduce the pain of coding a responsive email. Its semantic syntax makes it easy and straightforward and its rich standard components library speeds up your development time and lightens your email codebase. MJML’s open-source engine generates high quality responsive HTML compliant with best practices.

It seems that the language is basically HTML, but with certain elements replaced by custom elements. E.g., <hr> is replaced by <mj-devider> and <p> becomes <mj-text>. Raw HTML can be embedded within most elements.

There's some interest to have this as an output format, and I'd like to gauge interest for a full Haskell writer for this format (instead of a custom Lua writer). Would this be a worthy addition?