New: Add postfix fixed ordering option

- With the change to camelCase, it is easier to extend the usageof
  optiosn now, and in this case I've allowed the `orderFixed` (formally
  `aaSortingFixed` parameter to be passed in as an object as well as an
  array. As an object the `pre` and `post` parameters are used to
  specify prefix and / or postfix ordering. Useful for gouping.
Dev fix: row().child() checks for undefined now since an empty string
would be a valid input for a details row. Not a useful one, but
none-the-less a valid on.