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.
htsbuf_to_string() returns an allocated string so it must be free'd by the caller.

so something like

    char *s = htsbuf_to_string(&buf);
    event_dispatch(event_create_openurl(s, NULL, NULL, NULL, NULL, NULL));
    free(s);