Add .eliom extension for Ocsigen (OCaml web framework)

[Ocsigen](http://ocsigen.org/) is a web server and framework using OCaml.

It uses a syntax extension to manage the client-side, server-side or shared code. It also has its own compiler which requires a different file extension `.eliom`.

__OCaml syntax__ seems fine to correctly display the Ocsigen code.