No default connection string if connectManually

[rigwild]

This adds the abillity to provide null instead of a default connection string when connectManually option is turned on.

Vue.use(VueNativeSock, null, {
  store,
  connectManually: true
})

I got this use case for an app which can connect arbitrarily to the user's selected host.
Temporary workaround : set ws://example.com as a default connection string and use :

Vue.prototype.$socket.$connect('ws://your_host.com')

to connect to any WebSocket server. This workaround is not ideal as you don't own the example.com domain.