Add aliases to Kamal

Aliases are defined in the configuration file under the `aliases` key.

The configuration is a map of alias name to command. When we run the
command the we just do a literal replacement of the alias with the
string.

So if we have:

```yaml
aliases:
  console: app exec -r console -i --reuse "rails console"
```

Then running `kamal console -r workers` will run the command

```sh
$ kamal app exec -r console -i --reuse "rails console" -r workers
```

Because of the order Thor parses the arguments, this allows us to
override the role from the alias command.

There might be cases where we need to munge the command a bit more but
that would involve getting into Thor command parsing internals,
which are complicated and possibly subject to change.

There's a chance that your aliases could conflict with future built-in
commands, but there's not likely to be many of those and if it happens
you'll get a validation error when you upgrade.

Thanks to @dhnaranjo for the idea!

test/integration/main_test.rb

@@ -82,6 +82,19 @@ class MainTest < IntegrationTest
     assert_equal({ "cmd"=>"wget -qO- http://localhost > /dev/null || exit 1", "interval"=>"1s", "max_attempts"=>3, "port"=>3000, "path"=>"/up", "cord"=>"/tmp/kamal-cord", "log_lines"=>50 }, config[:healthcheck])
   end
 
+  test "aliases" do
+    @app = "app_with_roles"
+
+    kamal :envify
+    kamal :deploy
+
+    output = kamal :whome, capture: true
+    assert_equal Kamal::VERSION, output
+
+    output = kamal :worker_hostname, capture: true
+    assert_match /App Host: vm3\nvm3-[0-9a-f]{12}$/, output
+  end
+
   test "setup and remove" do
     # Check remove completes when nothing has been setup yet
     kamal :remove, "-y"