allow renaming of generated field

[alvinsj]

add a rename/map option to denomalizer definition to allow renaming of generated field.
e.g.:

denormalize :order do |order|
  order.denormalize :customer do |customer|
    customer.map :name, :customer_name
  end
end

[alvinsj]

i will be working on enabling custom prefix for denormalized fields, raise error if field with same names exist. e.g.

denormalize :order do |order|
  order.denormalize :customer, prefix: :cust # generated fields will be i.e. cust_first_name
end

[alvinsj]

pull request https://github.com/alvinsj/flatten_record/pull/16 for v1.0 include implementation of custom prefix