TODO: lib/plugins: make sure required attributes are set

[bcoles]

  def self.define(&block)
    # TODO: plugins should isolated
    p = new
    p.instance_eval(&block)
    p.startup
    # TODO: make sure required attributes are set
    Plugin.attributes.each { |symbol| p.instance_variable_get("@#{symbol}").freeze }
    Plugin.registered_plugins[p.name] = p
  end