Allow underscored strings and symbols for action dsl

This commit extends the controllers `.action` DSL to allow underscored strings and symbols along with the Ruby class name string option as follows:

```ruby
class ArticlesController
  include Lotus::Controller

  action 'Index' do # You may use valid Ruby class names as strings ...
    # ...
  end

  action 'show' do # ... or 'underscored' strings ...
    # ...
  end

  action :update_pricing do #  ... or symbols.
    # ...
  end
end

ArticlesController::UpdatePricing.new.call({})
```

It is just something I stumbled over briefly and somehow expected to work. I also think it is more legible, yet it might introduce some unwanted magic.