Feature: shell mode

[flowchartsman]

pulsar-shell is pretty cool and with a little work, pulsarctl could do it too, along with better autocomplete. I experimented with a preliminary implementation using cobra-shell, and the result was surprisingly functional. There are definitely a couple rough edges with cobra-ehsll, like error reporting, and autocomplete might need a deeper integration, but I thought I'd open this issue to explore the possibility and gauge interest.