Add array-of-tables to keygroup markup

The markup for array-of-tables at https://github.com/toml-lang/toml#array-of-tables is
[[ foo ]]

Right now, the last ] isn't part of the markup. Figured making this an explicit token is the most apparent way to fix this.

lib/ace/mode/toml_highlight_rules.js

@@ -57,6 +57,10 @@ var TomlHighlightRules = function() {
             regex : '"(?=.)',
             next  : "qqstring"
         },
+        {
+            token: ["variable.keygroup.toml"],
+            regex: "(?:^\\s*)(\\[\\[([^\\]]+)\\]\\])"
+        },
         {
             token: ["variable.keygroup.toml"],
             regex: "(?:^\\s*)(\\[([^\\]]+)\\])"