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/integration/main_test.rb

@@ -93,6 +93,9 @@ class MainTest < IntegrationTest
 
     output = kamal :worker_hostname, capture: true
     assert_match /App Host: vm3\nvm3-[0-9a-f]{12}$/, output
+
+    output = kamal :uname, "-o", capture: true
+    assert_match "App Host: vm1\nGNU/Linux", output
   end
 
   test "setup and remove" do