Expose the device's state (connected or disconnected)

[theckman]

If a UniFi device is offline, whether intentionally or accidentally, this
currently breaks the unifi_exporter. By extending this package, we can fix
that bug in unifi_exporter.

[theckman]

@mdlayher wanted to directly ping, since this will allow me to fix a bug triggering a panic in unifi_exporter. I've a wireless AP that's only connected sometimes (used for meshing), and when it's offline it breaks my stats collection because one of the stats structs is nil.

Would you be willing to have this merged into master? I could probably also take a look at the other PRs and see if they are applicable (if you wanted to give me write access to the repo as well). Alternatively, happy to fork the repo.