Add -N or --no-default-middleware option.

This would prevent Unicorn (Rainbows) from adding default middleware,
as if RACK_ENV were always none. (not development nor deployment)

This is implemented in Unicorn, so we only need to update
the option parser here.

Discussion thread on Unicorn mailing list:
http://rubyforge.org/pipermail/mongrel-unicorn/2013-January/001675.html

Signed-off-by: Eric Wong <normalperson@yhbt.net>