* Add common TLS config routines

These routines can be used to load TLS configs based upon the
args in the Corefile.

* Add common routine for Corefile arg handling

Add the NewTLSConfigFromArgs routine so that you can just pass
in the Corefile args and get a tls.Config. This ensures the
parameters are handled consistently across middleware.

* Change to varargs style params

Change to use args ...string instead of []string. Add
documentation of what each call means.