Consider looking up the public IP of the host system if no IP given

[nexdrew]

Could use Sindre's public-ip module.

This would make the CLI much more convenient - just install and run which-cloud on the host!

Not quite sure how this would be best represented in the API - perhaps just accept a callback as the first-and-only argument? That would be nicer than forcing a module to pass a falsy IP value, but slightly complicates the implementation, particularly if it ever accepts separate opts too.

I could imagine all of these being valid:

// callback
whichCloud(ip, opts, cb)
whichCloud(null, opts, cb) // do ip lookup
whichCloud(ip, cb)
whichCloud(cb) // do ip lookup
// promise
whichCloud(ip, opts).then(c => {})
whichCloud(null, opts).then(c => {}) // do ip lookup
whichCloud(ip).then(c => {})
whichCloud().then(c => {}) // do ip lookup

But we could, of course, just use another function for the "do IP lookup" option:

// callback
whichCloud.lookup(opts, cb)
whichCloud.lookup(cb)
// promise
whichCloud.lookup(opts).then(c => {})
whichCloud.lookup().then(c => {})