add nicer event declaration syntax #52

Currently to declare an event (i.e. a callback that can be attached via the .on method you need some complicated param declaration syntax like this:

param :on_saved, allow_nil: true, type: Proc, default: nil # declares the 'saved' event

Whew

Now you can say:

raises :saved

This will create a saved! method within your component to raise the event.

You can override the internal method name using the alias option:

`raises :saved, alias: :save_it # defines the save_it method instead of saved!

For non-standard event names you can use the same syntax as the .on method uses, i.e.

raises '<SAVE>', alias: saved! # you must supply an alias for non-standard names

To attach a SAVE handler you say `.on('') (as you do today)

For example here is a sample component (see #51 for why there are no more params. prefixes)

class EditItem 
  include Hyperstack::Component
  include Hyperstack::Observable
  param :todo
  others :others
  raises :save
  raises :cancel
  after_mount { ::Element[dom_node].focus }
  render do
    INPUT(
      @others,
      defaultValue: @todo.title,
      placeholder: "What is left to do today?",
      key: @todo
    )
    .on(:key_down) do |evt|
      next unless evt.key_code == 13
      @todo.update(title: evt.target.value)
      save!
    end
    .on(:blur) { cancel! }
  end
end