Add EEx (Embedded Elixir)

EEx is embedded Elixir: http://elixir-lang.org/docs/v1.0/eex/. Here's
some in-the-wild usage on Github: https://github.com/search?p=100&q=extension%3Aeex+NOT+nothack&ref=searchresults&type=Code&utf8=✓

This uses the "html_elixir" Ace mode, which was added in this pull
request on the Ace project: https://github.com/ajaxorg/ace/pull/2696

lib/linguist/languages.yml

@@ -1323,6 +1323,16 @@ HTML+Django:
   - htmldjango
   ace_mode: django
 
+HTML+EEX:
+  type: markup
+  tm_scope: text.html.eex
+  group: HTML
+  aliases:
+  - eex
+  extensions:
+  - .eex
+  ace_mode: html_eex
+
 HTML+ERB:
   type: markup
   tm_scope: text.html.erb