Open output files after forking

[prokotg]

Fixes #233 Opening output files before forking makes the script crash on Windows . I did not test the influence of this PR on other systems and python versions.

[attardi]

Watch out: the second argument to reduce_process() might be sys.stdout and not an OutputSplitter.

if out_file == '-':
    output = sys.stdout

Then it won't have method open_file(), that you call in reduce+process().

[prokotg]

fixed