Allow chef-zero to listen on Unix domain socket.

Puma has support for unix domain sockets.  This commit adds a -s
option to chef-zero that allows the user to specify a path rather than
an IP and port:

    > chef-zero -s /tmp/chef-zero.sock

Clients can then connect to this socket:

    > telnet -u /tmp/chef-server.sock
    Trying /tmp/chef-server.sock...
    Connected to (null).
    Escape character is '^]'.
    GET /nodes HTTP/1.1

    HTTP/1.1 200 OK
    Content-Type: application/json
    Content-Length: 3
    {
    }