Merge pull request #2 from kikidong/master

urlpreview 实现黑名单，过滤掉类似web.qq.com的标题提示
cookie
fix 死循环
update avhttp
判断是否打开
changelog
使用 BOOST_ASIO_CORO_REENTER 而不是简化的 reenter, 因为是头文件, 最好不要导致 yield 关键字干扰
fix No newline at end of file
add some usefull define
fix issue pointed out by jackarain
Merge pull request #4 from microcai/master

some misc update for avbot
use async_dir_walk_continue_handler
add async_foreach , and let async_dir_walk use async_foreach