code: function formatLinesWith (formatter, str) {
    return str
      .split('\n')
      .map(function (line) {
        return line.length ? formatter(line) : ''
      })
      .join('\n')
  }
docs: Run a transformation function over the lines within a string and return the
  result.
  
  @param {function} formatter Formatter function
  @param {string} str String to format
  @return {string}