Defines a module function that can fill an input with a value. The function name is derived by action_name.

When scoped to a collection it requires an element be passed to the action.

## Example

  # without a collection
  defmodule MyPage do
    use PageObject

    fillable :email, "input[type='email']"
  end

  # click form submit button
  MyPage.email("demo@example.com")

  # with a collection
  defmodule MyPage do
    use PageObject

    collection :things, ".thing" do
      fillable :email, "input[type='email']"
    end
  end

  # fill input of 0th item in things
  |> MyPage.Things.get(0)
  MyPage.Things.email("demo@example.com")