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/hosts/setup.go

@@ -17,12 +17,7 @@ import (
 
 var log = clog.NewWithPlugin("hosts")
 
-func init() {
-	caddy.RegisterPlugin("hosts", caddy.Plugin{
-		ServerType: "dns",
-		Action:     setup,
-	})
-}
+func init() { plugin.Register("hosts", setup) }
 
 func periodicHostsUpdate(h *Hosts) chan bool {
 	parseChan := make(chan bool)