Defines a module function that queries the attribute value of an element on an html page. The function name is derived by name. When scoped to a collection the function takes an element as an argument.

## Example

  #not in a collection
  defmodule MyPage do
    use PageObject

    attribute :email_placeholder, "placeholder", "input[type='email']"
  end

  # queries the value of placeholder attribute on `input[type='email']` on the current page
  MyPage.email_placeholder

  #in a collection
  defmodule MyPage do
    use PageObject

    collection :things, item_scope: ".thing" do
      attribute :email_placeholder, "placeholder", "input[type='email']"
    end
  end

  # queries the value of the placeholder attribute of the 0th ".thing input[type='email']"
  MyPage.Things.get(0)
  |> MyPage.Things.email_placeholder