code: function factory (factoryName, factory) {
    if (_.isFunction(factory)) {
      definitions[factoryName] = Definition(factoryName, factory)
    }
  
    return definitions[factoryName]
  }
docs: Getter/setter that create and register a Definition
  if factory is provided or return the existing one if not.
  When creating a Definition, also add a gulp task that runs
  every tasks created with this Definition.
  
  @param {String} factoryName
  @param {Function} [factory]
  @returns {Object} Definition