Enable tab-completion of commands.

[Anavros]

This is going to do with readline somehow. But entering a partial command as a user, as comm... and hitting tab should auto-complete that command. If there are ambiguities, cycle through each option one by one. Ideally, allow completion for any user command that has a known set of options, e.g.:

options = [
    "pick |spam|eggs|ham|:breakfast",
]
#> pick sp... --> pick spam