Expose directives in dnsserver to help external plugin developers (#1315)

This fix expose directives in dnsserver package, so that external
plugin developers could easily build customerized coredns+plugin
without changing the code base tree of coredns.

The following is an example that could bundle coredns+example,
in one simple file without modifying coredns codebase:

```
package main

import (
        _ "github.com/coredns/example"

        "github.com/coredns/coredns/coremain"
        "github.com/coredns/coredns/core/dnsserver"
)

var directives = []string{
        "example",
        "log",
        "errors",
        ...
        ...
        ...
        "whoami",
        "startup",
        "shutdown",
}

func init() {
        dnsserver.Directives = directives
}

func main() {
        coremain.Run()
}
```

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

core/dnsserver/register.go

@@ -22,7 +22,7 @@ func init() {
 	flag.StringVar(&Port, serverType+".port", DefaultPort, "Default port")
 
 	caddy.RegisterServerType(serverType, caddy.ServerType{
-		Directives: func() []string { return directives },
+		Directives: func() []string { return Directives },
 		DefaultInput: func() caddy.Input {
 			return caddy.CaddyfileInput{
 				Filepath:       "Corefile",