Default fields resource option

Add an option to the resource class to specify the default fields. Would be used by the request parser if the fields are not specified. Would allow a pared down fields set as the default, while still allowing the full field set if requested.

For example:

class CommentResource < JSONAPI::Resource
  attributes :body
  has_one :post
  has_one :author, class_name: 'Person'
  has_many :tags

  filters :body

  default_fields [:body, :post, :author]
end

Would skip the tags relationship, unless specified in the request.

Comments?

Start by locating the resource class option handling and the request parser mentioned in the issue. Trace how requested fields and relationships are selected, then determine how a default list should apply only when fields are absent. Done means resources can declare default fields, omitted relationships are excluded by default, and explicitly requested fields still work.