Relax formatter opts (#557)
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. 
thoughtbot/bamboo