When no Corefile is given, default to loading the whoami middleware on
the default port (2053).  Also add back the -port flag that allows you
to override the default port.

Further cleanup the startup messages and use caddy's OnStartupComplete()
to blurp out which zones and ports we have.  These can be suppressed
with the -quiet flag.

Normal startup:

miek.nl.:1053
miek.nl2.:1053
example.org.:1054
2016/09/17 20:41:19 [INFO] CoreDNS-001 starting
CoreDNS-001 starting

with the -quiet flag:

2016/09/17 20:41:34 [INFO] CoreDNS-001 starting