Reading depopts from OPAM file

Plus some revised documentation for DEPOPTS and HOOKS.
> What do you think?

You are right, the ability to just select some subsets of dependencies is a better idea. 

But then, do you think it makes sense to have one specific run without any dependencies and then another one with a subset of dependencies? Especially if there are many subsets tested, the one run without optional dependencies might be quite redundant. Or maybe I am just trying to overoptimize, in that case please stop me :smile:.