function CountPattern(params) {
    Transform.call(this, null, params);
  }
Count regexp-defined pattern occurrences in a text field.
@constructor
@param {object} params - The parameters for this operator.
@param {function(object): *} params.field - An accessor for the text field.
@param {string} [params.pattern] - RegExp string defining the text pattern.
@param {string} [params.case] - One of 'lower', 'upper' or null (mixed) case.
@param {string} [params.stopwords] - RegExp string of words to ignore.