Core: Enhance "include" command to process sub-directories recurisively

In the old code, when use "include" command in "mask" syntax, such as:

        include vhosts/*.conf

it'll only process files with ".conf" suffix in "vhosts/" directory.

With this patch, the "include" command will process all files with
".conf" suffix in "vhosts/" and its all sub-directories recurisively.

Signed-off-by: Yunkai Zhang <qiushu.zyk@taobao.com>