WIP - Allow custom scopes.

[nickurban]

Supports custom scopes, for example to make nested resources work.

1. Adds an extension point class method named custom_route_scopes to JSONAPI::Resource, which allows overriding the scopes used to locate the path helper.
2. If context[:path_parameter] is specified in the controller (via override of context method), this will pass the parameters into the path helper to specify the parent id path segments.

Given routes like this:

jsonapi_resouces :parents do
  jsonapi_relationships
  jsonapi_resources :children
end

Override context in the ChildrenController:

    def context
      { 
        parent_id: params[:parent_id],
        path_parameters: [ params[:parent_id] ],
      }
    end

And a custom ChildProcessor, that e.g. searches using the parent_id.

def load_parent
    parent_id = params[:context].try(:[], :parent_id)
    # TODO: <load the parent resource>
end

and override the show, create_resource etc. methods to call load_parent and work from there.