parallel: Add support for replacing {} with different arguments.

Add support in parallel for replacing {} with different arguments.
Now -i and -n are not mutually exclusive anymore.

Possible values for replace_cb:
  - 0: Don't replace {} (no -i specified).
  - 1: Replace all {} with the same argument (-i used without -n).
  - 2: Replace each {} with a different argument (-i and -n used).
       The number of {} strings that are replaced, depends on the
       number specified by -n.

The new behaviour of replacing {} with different arguments is only
enabled when -i and -n are specified.

Signed-off-by: Gert Hulselmans <hulselmansgert@gmail.com>