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/app.rb

@@ -71,11 +71,12 @@ class Kamal::Cli::App < Kamal::Cli::Base
     end
   end
 
-  desc "exec [CMD]", "Execute a custom command on servers within the app container (use --help to show options)"
+  desc "exec [CMD...]", "Execute a custom command on servers within the app container (use --help to show options)"
   option :interactive, aliases: "-i", type: :boolean, default: false, desc: "Execute command over ssh for an interactive shell (use for console/bash)"
   option :reuse, type: :boolean, default: false, desc: "Reuse currently running container instead of starting a new one"
   option :env, aliases: "-e", type: :hash, desc: "Set environment variables for the command"
-  def exec(cmd)
+  def exec(*cmd)
+    cmd = Kamal::Utils.join_commands(cmd)
     env = options[:env]
     case
     when options[:interactive] && options[:reuse]

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

lib/kamal/utils.rb

@@ -77,4 +77,8 @@ module Kamal::Utils
   def stable_sort!(elements, &block)
     elements.sort_by!.with_index { |element, index| [ block.call(element), index ] }
   end
+
+  def join_commands(commands)
+    commands.map(&:strip).join(" ")
+  end
 end