md_future_symbol
returns all future symbols that provided by sina finance, see details on http://vip.stock.finance.sina.com.cn/quotes_service/view/qihuohangqing.html or http://vip.stock.finance.sina.com.cn/mkt/#global_qh)
md_future_symbol(...)
ignored parameters
if (FALSE) {
sybs = md_future_symbol()
}