Add support for output files

Add support for output files

Input and output files are mapped as follows:
(* represents the `else' condition, usually - and/or NULL)

     Provided    |      Used
  Input : Output |  Input : Output
  ------+--------+--------+--------
  in.ct : out.c  |  in.ct : out.c
  in.ct : -      |  in.ct : stdout
  in.ct : NULL   |  in.ct : in.c
  *     : out.c  |  stdin : out.c
  *     : *      |  stdin : stdout