Add `tns devices` as synonym for `tns device`

It would be nice if the command `tns devices` worked as a synonym for `tns device`. 

Justification: the equivalent Android adb command is `adb devices` and as a developer working on both NativeScript and Native Android apps this discrepancy is disconcerting / adds needless mental overhead.

Ref: https://developer.android.com/studio/command-line/adb.html#devicestatus
