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/trace/setup.go

@@ -11,12 +11,7 @@ import (
 	"github.com/caddyserver/caddy"
 )
 
-func init() {
-	caddy.RegisterPlugin("trace", caddy.Plugin{
-		ServerType: "dns",
-		Action:     setup,
-	})
-}
+func init() { plugin.Register("trace", setup) }
 
 func setup(c *caddy.Controller) error {
 	t, err := traceParse(c)