Add the ability to associate arel table alias nodes to a search model

[sholden]

Add the ability to associate Arel::Nodes::TableAlias objects to a PgSearch::Model. This allows us to build an Arel AST directly to allow searching across relations where ActiveRecord won't support joins (in our use case and in the specs, this involves a polymorphic association), but you could imagine many more use cases where this feature could make the PgSearch::Model configuration much more flexible and extensible.

Example from spec:

model do
  include PgSearch::Model
  has_many :other_models, :class_name => 'AssociatedModelWithHasManyAndPolymorphicPet', :foreign_key => 'ModelWithHasMany_id'

  def self.associated_pet_names
    cat_table = AssociatedCat.arel_table
    dog_table = AssociatedDog.arel_table
    bt_table = AssociatedModelWithHasManyAndPolymorphicPet.arel_table

    cat_join = bt_table.join(cat_table).on(bt_table[:polymorphic_pet_id].eq(cat_table[:id]).and(bt_table[:polymorphic_pet_type].eq('AssociatedCat')))
                   .project(bt_table[:ModelWithHasMany_id].as('id'), cat_table[:cat_name].as('name'))
    dog_join = bt_table.join(dog_table).on(bt_table[:polymorphic_pet_id].eq(dog_table[:id]).and(bt_table[:polymorphic_pet_type].eq('AssociatedDog')))
                   .project(bt_table[:ModelWithHasMany_id].as('id'), dog_table[:dog_name].as('name'))

    union = cat_join.union(dog_join)
    Arel::Nodes::TableAlias.new(union, 'associated_pet_names')
  end

  pg_search_scope :with_associated,
                  against: :title,
                  associated_against: {
                      other_models: [:title]
                  },
                  associated_arel: {
                      associated_pet_names => :name
                  }
end