Implement the -q or --quiet flag to add with minimal output

This implements issue #304.

It was done as much as possible in the same way as the
-r or --recursive flag.

License: MIT
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Here the Request instance was already successfully initialized, we need to still return it:
```golang
   return req, nil, nil, u.ErrCast()
```

(see https://github.com/jbenet/go-ipfs/pull/378)