Handle resource plugin

I've added the feature to handle resource plugin. If itamae fails to get constant `Itamae::Resource::Foo`, it tries to get `Itamae::Plugin::Resource::Foo` .

Also I've moved mail alias resource to [external gem](https://rubygems.org/gems/itamae-plugin-resource-mail_alias) to test this feature.

It works fine on my environment with this recipe.

```ruby
require 'itamae/plugin/resource/mail_alias'

mail_alias 'foo' do
  recipient 'bar'
end
```

Please check it.