function parseModifier (ruleName) {
  if (/^[\^]/.test(ruleName.charAt(0))) {
    return [camel(ruleName.substr(1)), ruleName.charAt(0)]
  }
  return [camel(ruleName), null]
}
Return the rule name with the applicable modifier as an array.
@param {string} ruleName
@return {array} [ruleName, modifier]