httpd-host type changed string to allow easily use of different host names

When httpd-host is used as symbol then it's hard to pass different host names or ip addresses to make-network-process, which expects them to be strings.

Added 'cl dep to tests, without which they don't work in batch mode - no "flet" definition.

Created a helper function which asks user for directory and starts the web server with the dir as current httpd-root. I find it very helpful, especially when working on different project with skewer-mode.