Extensions homogenization

This PR is in relation with ocsigen/ocsigenserver#41 - Eliom is also an extension. Perhaps there is a problem with the documentation (I don't know all the intricacies of Eliom), if you could see.

There is a factorization between the function of parsing the configuration file and the `init` function. Of course, the request scheduling of Eliom on Ocsigenserver is not yet programmed, it will be another PR for statically linking between Eliom and Ocsigenserver (and MirageOS, oups).
`Eliommod_gc` functions already take an option, no need to double unwrap.