Add a hook to access the underlying Active Record relation

[One of the examples](https://github.com/cerebris/jsonapi-resources#finders) in the readme shows overriding the `find` method to achieve scoping resources to current user:

```ruby
def self.find(attrs, options = {})
  ...
  authors = context.current_user.find_authors(attrs)
  ...
end
```

I believe this a very common use case in API's. The problem is that for this to really work one must also override `find_by_key` and `find_by_keys`.

One way to make this easier is to provide a hook to override the Active Record relation:

```ruby
class PostResource < JSONAPI::Resource
  attribute :id, :title, :body

  # instead of overriding find, find_by_key and find_by_keys
  def self.relation(options = {})
    context = options[:context]
    context.current_user.posts
  end
end
```

This could also be used for something like pagination:

```ruby
class PostResource < JSONAPI::Resource
  attribute :id, :title, :body

  def self.relation(options = {})
    context = options[:context]
    _model_class.paginate(page: context.params[:page])
  end
end
```


_p.s.:_ The implementation is just a quick example and would need tests and documentation (and maybe a better name for the hook) if you think this is a viable approach.