Define hooks from within the bot instance

[dblock]

Coming from #154

It would be nice if some common hooks could be defined alongside command, e.g. :

  class MyBot < SlackRubyBot::Bot
    command 'ping' do |client, data|
      client.say(text: 'pong', channel: data.channel)
    end

    hello do |client, data|
      ...
    end
    
    # or
    hook 'hello' do |client, data|
      ...
    end
  end