Added custom ArgumentParser (supports allow_abbrev)
    
    Python 2's argparse.ArgumentParser do not support allow_abbrev option
    that is implemented in Python 3's argparse.ArgumentParser; the custom
    ArgumentParser class implements the allow_abbrev option functionality
    for Python 2.
    
    The abbreviation of passed arguments is switched off (allow_abbrev=False).
    
    Discussion: https://pagure.io/fedpkg/pull-request/186
    
    Signed-off-by: Jiri Kucera <jkucera@redhat.com>