Add ".es" to recognised ECMAScript extensions

* Add ".es" to recognised JavaScript extensions
* Add heuristic to differentiate Erlang from ECMAScript
* Add test-case for .es heuristic

test/test_heuristics.rb

@@ -83,6 +83,13 @@ class TestHeuristcs < Minitest::Test
       "ECLiPSe" => all_fixtures("ECLiPSe", "*.ecl")
     })
   end
+  
+  def test_es_by_heuristics
+    assert_heuristics({
+      "Erlang" => all_fixtures("Erlang", "*.es"),
+      "JavaScript" => all_fixtures("JavaScript", "*.es")
+    })
+  end
 
   def test_f_by_heuristics
     assert_heuristics({