Relax formatter opts (#557)

closes #499 

What changed?
============

`Bamboo.Formatter:format_email_address` allows for ignoring `_opts` 
(the second argument). But the it throws a type error after Elixir 1.9.1 
if no options are passed because the typespec requires a map with
`:type`. This commit relaxes that typespec constraint to make the 
`:type` optional.

lib/bamboo/formatter.ex

@@ -63,7 +63,7 @@ defprotocol Bamboo.Formatter do
   the address.
   """
 
-  @type opts :: %{type: :from | :to | :cc | :bcc}
+  @type opts :: %{optional(:type) => :from | :to | :cc | :bcc}
 
   @spec format_email_address(any, opts) :: Bamboo.Email.address()
   def format_email_address(data, opts)