Recognize Elixir source files automatically

core/prelude-packages.el

@@ -127,6 +127,8 @@ PACKAGE is installed only if not already present.  The file is opened in MODE."
     ("\\.csv\\'" csv-mode csv-mode)
     ("\\.d\\'" d-mode d-mode)
     ("\\.dart\\'" dart-mode dart-mode)
+    ("\\.ex\\'" elixir-mode elixir-mode)
+    ("\\.exs\\'" elixir-mode elixir-mode)
     ("\\.erl\\'" erlang erlang-mode)
     ("\\.feature\\'" feature-mode feature-mode)
     ("\\.go\\'" go-mode go-mode)