Brotli compression

[vouillon]

It would be nice to improve the deflatemod extension to support Brotli compression as well (with customizable compression settings, as Brotli can be slow).

[vasilisp]

If anybody is planning to do enhancements like these, please work against the cohttp branch (#124). Otherwise we will have trouble merging drastically incompatible changes.

[dinosaure]

A binding with brotli already exists. So, I know, this is put a new dependence in ocsigenserver but it will be good to use it.

Then, I take this opportunity to talk about decompress. For a mirage support, it will be good to use a full implemented compression in OCaml (and ocaml-git switchs recently to this library instead camlzip).