add malli experimental describe functionality

[escherize]

This follows many conventions in the JSON Schema tranformation ns.

I believe it is complete, meaning it can translate all included malli schemas.

It also exposes the ability to add a :description property which will be used instead of a computed (or un-computable) description string.

[ikitommi]

I'll check this tomorrow.

[ikitommi]

This is beautiful. Thanks!!

[ikitommi]

post-stylize: https://github.com/metosin/malli/commit/9877a8b918f7dd34194ccef8337eed9c2433ac6a (will later write a style-guide)