Implement `[]` operator for direct access to elements of a struct

[michael-projectx]

A struct:

struct foo {
  key = 1

  struct bar {
    key = 2
  }
}

Should be accessible via cfg['foo']['key'] or cfg['foo']['bar']['key']

[michael-projectx]

Start out by making this only work for objects of type flexi_cfg::Reader. That should be pretty straight-forward