Add plugin/open url

This commit add plugin/open url handler, that makes json from request arguments.
Example:  http://showtime_ip:42000/plugin/open/some/plugin?arg0=val0&arg1=val2
will be transformed into some/plugin:{"arg0":"val0","arg1":"val2"} and invoked in showtime.
I am creating VK plugin for showtime. I need this url for performing oauth authorization with user's smartphone. Old handler /?url= doesn't suit me. I have described here https://showtimemediacenter.com/boards/6/topics/8783 why.
I see. I was thinking, that string pointer is part of buf, and it will be deallocated with htsbuf_queue_flush.