Change default, --lookup-extents=no

There's too many caveats with this option right now to default it on. I
think it's better for the users to only turn it on when it might help. Once
we're looking up extent owners we can default this back to yet.

Signed-off-by: Mark Fasheh <mfasheh@suse.de>
Take some of the less obvious options out of usage() and point folks to the
man page for them instead. This makes the output of 'duperemove --help' more
concise while also giving users a pointer to where they can read a lot more
about duperemove.

Signed-off-by: Mark Fasheh <mfasheh@suse.de>
Update README

Signed-off-by: Mark Fasheh <mfasheh@suse.de>