Adding eliomdoc (experimental)

I've implemented ```eliomdoc```. It works like ```eliomc, eliomopt, etc..```. It is a wrapper which forwards the good package and extension syntax files. I've also implemented ```eliompp``` which is a little preprocessor which removes specific sections. I made this tool because ```camlp4``` does not include commentaries into the AST and so we can't select commentaries from a specific section.

These tools are experimental, I've noticed some bugs with ```camlp4```, but it is usable for now. I use it for ```eliom-base-app``` and I'm gonna use it for ```eliom-widgets``` too.