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/zdirectives.go

@@ -9,8 +9,7 @@ package dnsserver
 // feel the effects of all other plugin below
 // (after) them during a request, but they must not
 // care what plugin above them are doing.
-
-var directives = []string{
+var Directives = []string{
 	"tls",
 	"nsid",
 	"root",