Add `-disabled` flag for new proxies in CLI

[xthexder]

Right now it's not possible to create proxies from the cli with an initially disabled state, while it is possible with CURL and I believe most of the clients.

This is a relatively niche use-case, but it was brought up here: https://github.com/Shopify/toxiproxy/issues/148#issuecomment-267102543 Should be easy to add in an upcoming version.

[sirupsen]

👍