all: simply registering plugins (#3287)

Abstract the caddy call and make it simpler.

See #3261 for some part of the discussion.

Go from:

~~~ go
func init() {
       caddy.RegisterPlugin("any", caddy.Plugin{
               ServerType: "dns",
               Action:     setup,
       })
}
~~~

To:

~~~ go
func init() { plugin.Register("any", setup) }
~~~

This requires some external documents in coredns.io to be updated as
well; the old way still works, so it's backwards compatible.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/register.go

@@ -0,0 +1,11 @@
+package plugin
+
+import "github.com/caddyserver/caddy"
+
+// Register registers your plugin with CoreDNS and allows it to be called when the server is running.
+func Register(name string, action caddy.SetupFunc) {
+	caddy.RegisterPlugin(name, caddy.Plugin{
+		ServerType: "dns",
+		Action:     action,
+	})
+}