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\""
```

lib/kamal/cli/server.rb

@@ -1,7 +1,8 @@
 class Kamal::Cli::Server < Kamal::Cli::Base
   desc "exec", "Run a custom command on the server (use --help to show options)"
   option :interactive, type: :boolean, aliases: "-i", default: false, desc: "Run the command interactively (use for console/bash)"
-  def exec(cmd)
+  def exec(*cmd)
+    cmd = Kamal::Utils.join_commands(cmd)
     hosts = KAMAL.hosts | KAMAL.accessory_hosts
 
     case