Make Field::Select more flexible

• This PR adds more flexibility to Field::Select so we can set an array of arrays and a hash to collection option. This behavior would be more intuitive since it follows Rails form helper convention.
• This also solves the absence of Enum field referred to #322 and #533 (closed) so we don't have to create a Enum field as a custom or built-in field anymore.
• Added select_spec.rb so we can test Select field

By default, it displays label data(e.g. "Pending") instead of raw persisted data(e.g. 1) on show/index page. Search feature would be an issue because users would want to search by label data but the system searches only by persisted data for the moment. However, that issue should be solved by another PR, not this one. Thank you for this awesome gem 😃

Basic usage:

status: Field::Select.with_options(collection: [ ['Pending',1], ['Approved',2], ['Rejected',3] ])

=>

<select name="post[status]" id="post_status">
  <option selected="selected" value="1">Pending</option>
  <option value="2">Approved</option>
  <option value="3">Rejected</option>
</select>

For Enum:

status: Field::Select.with_options(collection: Post.localized_statuses.invert)

class Post < ApplicationRecord
  enum status: { submitted: 0, approved: 1, rejected: 2 }
  def self.localized_statuses
    I18n.t 'activerecord.attributes.post/status'
  end
  # or
  # def self.localized_statuses
  #   Post.statuses.keys.map {|k| [k, Post.human_attribute_name("post.status.#{k}")] }
  # end
end

activerecord:
  attributes:
    post/status:
      submitted: "承認待ち"
      approved: "承認済み"
      rejected: "取り消し"