Mix format (#232)

Since the minimum Elixir version is 1.7+, we should mix format the
the whole module.

Author: kianmeng

.formatter.exs

@@ -0,0 +1,4 @@
+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]