Add clean way to specify serializer options

Instead of writing this:

```ruby
class PostSerializer < Encore::Serializer::Base
  def self.can_include
    [:project, :user]
  end

  def self.can_access
    [:project]
  end

  def self.root_key
    :user_posts
  end
end
```

We can do this:

```ruby
class PostSerializer < Encore::Serializer::Base
  can_include :project, :user
  can_access :project
  root_key :user_posts
end
```