Subscript syntax for Variables (#215)

* Implement variable indirect resolution

* Add some tests

* Changelog entry

* Update documentation

* Rework the syntax to use brackets instead of a $

* Move the lookup parser into it's own file

* Add invalid syntax tests

* Swift 3 support

* Rename some things + extra test

docs/templates.rst

@@ -31,6 +31,24 @@ For example, if `people` was an array:
     There are {{ people.count }} people. {{ people.first }} is the first
     person, followed by {{ people.1 }}.
 
+You can also use the subscript operator for indirect evaluation. The expression
+between brackets will be evaluated first, before the actual lookup will happen.
+
+For example, if you have the following context:
+
+.. code-block:: swift
+
+    [
+      "item": [
+        "name": "John"
+      ],
+      "key": "name"
+    ]
+
+.. code-block:: html+django
+
+    The result of {{ item[key] }} will be the same as {{ item.name }}. It will first evaluate the result of {{ key }}, and only then evaluate the lookup expression.
+
 Filters
 ~~~~~~~