code: function filterDeclarations(selectedRule) {
      return function (allowedDeclarationsList, attributeObject) {
        // If this property is whitelisted...
        if (selectedRule.hasOwnProperty(attributeObject.prop)) {
          var matchesRegex = selectedRule[attributeObject.prop].some(function(regularExpression) {
            return regularExpression.test(attributeObject.value);
          });
  
          if (matchesRegex) {
            allowedDeclarationsList.push(attributeObject);
          }
        }
        return allowedDeclarationsList;
      };
    }
docs: Filters the existing attributes for the given property. Discards any attributes
  which don't match the whitelist.
  
  @param  {object} selectedRule             - Example: { color: red, font-family: helvetica }
  @param  {array} allowedDeclarationsList   - List of declarations which pass whitelisting.
  @param  {object} attributeObject          - Object representing the current css property.
  @property {string} attributeObject.type   - Typically 'declaration'.
  @property {string} attributeObject.prop   - The CSS property, i.e 'color'.
  @property {string} attributeObject.value  - The corresponding value to the css property, i.e 'red'.
  @return {function}                        - When used in Array.reduce, will return an array of Declaration objects