Subcommand aliases

This isn't a blocker for 1.0 since it's achievable with external commands, but: it'd be nice to support an `[[aliases]]` section in the config that allows users to map built-in subcommands to their own aliases.

For example, this feature would allow a user to alias `kbs2 list` to `kbs2 ls` without having to write a `kbs2-ls` script.