support -n option

support -n option

This makes parallel stuff more than one argument to the subcommands.
It's very useful when subcommands are slow to start and are able to
process multiple files at once, that tasks are small but plenty.

For example:
    $ ./parallel -n 3 echo -- a b c d e f g h
    a b c
    d e f
    g h

Signed-off-by: Pierre Habouzit <madcoder@debian.org>