Driver: Read a config file to apply the right -P and -L.

[panglesd]

This PR makes the driver follow a convention to know which packages/libraries to include in -P/-L during linking, for opam-installed libraries. (This convention is what I understood that we proposed in various dev meetings. However, I have a slightly different proposition, see at the end of the post).

The file has to be at path <opam_root>/doc/<pkgname>/odoc-config.sexp.

It has to be a valid sequence of whitespace-separated s-expression. Each s-expression must be of the form '(libraries <atom list>) or (packages <atom list>).
The atoms present in each field correspond respectively to the -L and -P to add to the linking of the pages and modules of the package. That is, atoms in 'library must be library names, while atoms in packages must be package names.

I tried to add cram tests for the driver, but failed miserably. I thus included instructions on how to run tests.

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I think specifying pages at a package level, and libraries at a library level, is confusing for the user and does not add much value.
I would propose to have a single (deps <package list>) stanza which adds:

• a -P for each package listed
• a -L for each library of each package listed

This is less "fine-grained" but less confusing I think. Let me know if there is some advantages to being able to specify dependencies at the library granularity!

[Julow]

Looks good.

[Julow]

These will be used for both pages and modules ? I think that's fine. My initial idea was that the new dependencies will be specified on the (documentation) stanza and it might be weird that they also affect modules. It seems that this will be defined in dune-project instead, which is fine.