Adds `kamal alias`: list existing aliases

[supercobra]

kamal alias lists existing aliases from deploy.[target].yml files.

Example:

$ kamal alias -d production                                                                   
console                        app exec --interactive --reuse "bundle exec rails console"     
shell                          app exec --interactive --reuse "bash"                          
logs                           app logs -f                                                    
dbc                            app exec --interactive --reuse "bundle exec rails dbconsole"