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

grammars.yml

@@ -248,6 +248,7 @@ vendor/grammars/elixir-tmbundle:
 - source.elixir
 - text.elixir
 - text.html.elixir
+- text.html.eex
 vendor/grammars/erlang.tmbundle:
 - source.erlang
 - text.html.erlang.yaws