Additionally warn on any ALTER statement
Long overdue CLI revamp; now subcommands are explicitly managed

This is a minimal working version and is poorly factored.  In particular, single/multiple results aren't modeled and neither is the distinction between what's valid on a Statement and a Relation (but it's better than what was there before)
Move args and _warn into Hive backend and use it to clean up all run methods
Thread cluster label into Qubole backend