First draft on multi-version support

[EmileTrotignon]

This a rough draft for multi-version support for ocamlformat. It use first class modules to pick the specified version of ocamlformat.

There are two main limitations with the design :

• First of all, the configuration type is shared across all version. To fix this would probably require modular explicit.
• Second, the configuration cmdliner terms are also shared across all version. This is probably easier to fix, it just requires some hacks (we need to fetch the version number before parsing any other config).

I would really like comments on the overall design, I do not think this is ready for merging at all. If the design of the patch looks good to everyone, then a lot of cleaning up and some testing would be required.