Add --with-dns option to weave run

Change the script so it doesn't set any Docker DNS options unless `--with-dns` is specified.
`--with-dns` can go either before or after the CIDR on the `weave run` command.
We still call `tell_dns` on every container, so it can be registered in weavedns even though it is not actually using weavedns.

Fixes #208
This bit reads like "if you supply --with-dns, ..., and both weave run and weave attach ..", whereas weave run and weave attach will register regardless.