Allow multiple arguments for exec commands

If you can have an alias like:

```
aliases:
  rails: app exec -p rails
```

Then `kamal rails db:migrate:status` will execute
`kamal app exec -p rails db:migrate:status`.

So this works, we'll allow multiple arguments `app exec` and
`server exec` to accept multiple arguments.

The arguments are combined by simply joining them with a space. This
means that these are equivalent:

```
kamal app exec -p rails db:migrate:status
kamal app exec -p "rails db:migrate:status"
```

If you want to pass an argument with spaces, you'll need to quote it:

```
kamal app exec -p "git commit -am \"My comment\""
kamal app exec -p git commit -am "\"My comment\""
```

test/fixtures/deploy_with_aliases.yml

@@ -18,3 +18,4 @@ aliases:
   info: details
   console: app exec --reuse -p -r console "bin/console"
   exec: app exec --reuse -p -r console
+  rails: app exec --reuse -p -r console rails