* Move *proxy* to external

move the proxy plugin into coredns/proxy and remove it as a default
plugin. Link the proxy to deprecated in plugin.cfg

coredns/proxy doesn't compile because of the vendoring :(
Signed-off-by: Miek Gieben <miek@miek.nl>

* Add github.com/coredns/proxy
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>