whole_func_string: function getAllSelectors(css) {
      let selectors = [];
      css.walkRules((rule) => {
          selectors = _.concat(selectors, rule.selector);
      });
      return selectors;
  }
func_code_string: function getAllSelectors(css) {
      let selectors = [];
      css.walkRules((rule) => {
          selectors = _.concat(selectors, rule.selector);
      });
      return selectors;
  }
func_documentation_string: Get all the selectors mentioned in {css}
  @param  {Object} css        The postcss.Root node
  @return {Array}