Add aliases to Kamal

Aliases are defined in the configuration file under the `aliases` key.

The configuration is a map of alias name to command. When we run the
command the we just do a literal replacement of the alias with the
string.

So if we have:

```yaml
aliases:
  console: app exec -r console -i --reuse "rails console"
```

Then running `kamal console -r workers` will run the command

```sh
$ kamal app exec -r console -i --reuse "rails console" -r workers
```

Because of the order Thor parses the arguments, this allows us to
override the role from the alias command.

There might be cases where we need to munge the command a bit more but
that would involve getting into Thor command parsing internals,
which are complicated and possibly subject to change.

There's a chance that your aliases could conflict with future built-in
commands, but there's not likely to be many of those and if it happens
you'll get a validation error when you upgrade.

Thanks to @dhnaranjo for the idea!

test/integration/docker/deployer/app_with_roles/.kamal/hooks/pre-connect

@@ -1,8 +1,4 @@
 #!/bin/sh
 
 echo "About to lock..."
-if [ "$KAMAL_HOSTS" != "vm1,vm2,vm3" ]; then
-  echo "Expected hosts to be 'vm1,vm2,vm3', got $KAMAL_HOSTS"
-  exit 1
-fi
 mkdir -p /tmp/${TEST_ID} && touch /tmp/${TEST_ID}/pre-connect

test/integration/docker/deployer/app_with_roles/config/deploy.yml

@@ -37,3 +37,6 @@ accessories:
       - web
 stop_wait_time: 1
 readiness_delay: 0
+aliases:
+  whome: version
+  worker_hostname: app exec -r workers -q --reuse hostname