Numerical ordering and abstract collecting

* fragments are now order by the :order _number_ instead
  of an alpha based ordering (also available in the actual
  concat module from R.I.P.)
* fragments can now use an abstract target name which is mapped
  to the name of the file_concat type (inspired by Richard Pijnenburg
  and then used the same approach as the actual concat module is
  using):
  * file_fragment's +path+ is now deprecated and +target+ should be used