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/cli/main_test.rb

@@ -566,6 +566,13 @@ class CliMainTest < CliTestCase
     end
   end
 
+  test "append to command with an alias" do
+    run_command("rails", "db:migrate:status", config_file: "deploy_with_aliases").tap do |output|
+      assert_match "docker exec app-console-999 rails db:migrate:status on 1.1.1.5", output
+      assert_match "App Host: 1.1.1.5", output
+    end
+  end
+
   private
     def run_command(*command, config_file: "deploy_simple")
       with_argv([ *command, "-c", "test/fixtures/#{config_file}.yml" ]) do