Allows additional cache options

This pull request allows to pass additional options when cache fetching. You only have to add cache_options method in your serializer to be able to add parameters as expires_in, race_condition_ttl, ...

```ruby
   class ProgrammerSerializer < ActiveModel::Serializer
      cached true
      attributes :name, :skills

      def self.to_s
        'serializer'
      end

      def cache_key
        object.name
      end

      def cache_options
        { race_condition_ttl: 10, expires_in: 1.minute }
      end
    end
```