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!

lib/kamal/configuration/validator/alias.rb

@@ -0,0 +1,15 @@
+class Kamal::Configuration::Validator::Alias < Kamal::Configuration::Validator
+  def validate!
+    super
+
+    name = context.delete_prefix("aliases/")
+
+    if name !~ /\A[a-z0-9_-]+\z/
+      error "Invalid alias name: '#{name}'. Must only contain lowercase letters, alphanumeric, hyphens and underscores."
+    end
+
+    if Kamal::Cli::Main.commands.include?(name)
+      error "Alias '#{name}' conflicts with a built-in command."
+    end
+  end
+end