Feature magicscope

Added the magic scope feature: _top and _parent (only if you enable it by setting blitz.enable_magic_scope = 1 in the ini)

The following should work:
```
$data = [ "users" => [ [ "name" => "john", "id" => 1 ], [ "name" => "bart", "id" => 2 ] ], "currentUser" => 2 ];

{{ BEGIN users }}
  name: {{ name }} {{ IF id == _parent.currentUser }} That's me! {{ END }}
{{ END users }}
```
_parent goes up in the stack and _top goes to the beginning of the stack.
Check tests/scope4.phpt for a more detailed example.