Make UI class use cmd under the hood

[cyberrumor]

Didn't know about this package before but it looks like it was written specifically for interactive shells. As such, it probably has a bunch of logic that does fairly similar things to the UI class. We could most likely remove a bunch of our own code in favor of a standard library solution.
https://docs.python.org/3/library/cmd.html#cmd.Cmd.cmdloop