Provide a translation module between html and js attributes

[Drup]

Currently, this translation module is hardcoded in js_of_ocaml and is quite partial (according to @hhugo). Given the new reflect mechanism we have in tyxml, it seems like it would be either to maintain inside tyxml directly.

I would tend to propose the following:

• Add [@reflect.js "foo"] annotations.
• Use a reflect-like mecanism to provide bi-directional dictionnaires.
• Allow either the html or the js attributes for the ppx.

[Drup]

@hhugo I'm doing some things around the ppx again. Do you think this is worth the effort ?