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/cli/cli_test_case.rb

@@ -63,4 +63,12 @@ class CliTestCase < ActiveSupport::TestCase
 
       assert_match expected, output
     end
+
+    def with_argv(*argv)
+      old_argv = ARGV
+      ARGV.replace(*argv)
+      yield
+    ensure
+      ARGV.replace(old_argv)
+    end
 end