Aka: add 'list' command & update listCommands override

Closes ProgVal/Limnoria#572. 

This PR adds a `list` command for viewing aliases and modifies the listCommands override (`list Aka`) to only display Aka's built-in commands. This should make the plugin easier to use, as `list Aka` won't be filled with  irrelevant alias definitions.

Also, remove an unused `import operator`.