Add function calls for variables

This allows for functions to be called, currently with a single
variable optionally passed.

This also adds a check to make sure that variable literals are
supported.

tatsu_grammar.txt

@@ -87,7 +87,7 @@ variable_identifier
 
 variable_accessor
     =
-    variable_accessor_brackets | variable_accessor_dot
+    variable_accessor_brackets | variable_accessor_call | variable_accessor_dot
     ;
 
 variable_accessor_brackets
@@ -95,6 +95,11 @@ variable_accessor_brackets
     "[" variable_identifier "]"
     ;
 
+variable_accessor_call
+    =
+    "(" [variable_identifier] ")"
+    ;
+
 variable_accessor_dot
     =
     "." variable_identifier

test_template.jinja

@@ -3,8 +3,12 @@
 {%block single key=val param=value %}
 test {{var}}
 {%endblock%}
+{% for item in dict_var.values() %}
+    <li>{% block loop_item %}{{ item }}{% endblock %}</li>
+{% endfor %}
 {# comment contents
 across lines #}
 {% raw %}
     {% block %}fake content{{ fake vars }}
-{% endraw %}
+{% endraw %}
+{{ '{{' }}