More fields in @doc-json

[dlesbre]

As mentioned in the discuss.ocaml post, I would appreciate a few extra fields in the generated JSON files to make integrating them in my website easier. jonludlam suggested I open an issue for these here as well:

• A title field with the page title (currently obtainable from breadcrumbs[-1].name
• A package field with the name of the current package (currently can be obtained from the path of the generated doc)
• A version field with the version of the current package (I'm building versioned docs). Currently I can only obtain this through git ref names.
• An objects fields with the list and type (val, module, type, module type...) of all objects defined in the current file
• A sherlodoc field indicating whether or not sherlodoc was present (and if so, the relative path from the current file to the db.js file).