Add "option" feature

[danielberkompas]

You can now simultaneously add <Say> verbs to Twiml and record those options for future use. For example:

people = Enum.with_index(people)
{opts, xml} = twiml do
  Enum.each people, fn({person, num}) ->
    option num, "Press #{num} for #{person.name}", then: :do_this
  end
end

opts # => [{1, [then: :do_this]}, ...]

This is very useful when combining ExTwiml with a state machine, say, to manage a phone tree.