Ocamlbuild plugin

I did a small ocamlbuild plugin, it uses cppo on files `.cppo.ml` to produce `.ml`. The documentation is in the Readme.

It seems to work nicely (see [this](https://github.com/Drup/LILiS/blob/cppo/myocamlbuild.ml#L14)), few remarks: 
* I need to test it a bit more.
* I may add a test/example.
* there is no tag for the -x option, I will add it later.
* I'm not convinced by the name choices for the tags, if you have better ideas, I will happily change the names.
* the opam file need to be changed, so I added it directly in the repository (which allows pinning).
* I'm completely terrible at makefiles, I have no idea how they work and I'm not sure I want to spoil my brain with this knowledge. I put together something using pieces of js_of_ocaml's makefile, it works, but I will be very happy if you want to rewrite it differently.

cc @hhugo