middleware/root: add it (#330)

This PR adds the *root* middleware that specifies a path where
all zone file (the *file* middleware is the only consumer now) can
be found. It works the same as in Caddy.

Documentation can be found in the README.md of the middleware.

Fixes #307

middleware/root/root.go

@@ -0,0 +1,42 @@
+package root
+
+import (
+	"log"
+	"os"
+
+	"github.com/miekg/coredns/core/dnsserver"
+
+	"github.com/mholt/caddy"
+)
+
+func init() {
+	caddy.RegisterPlugin("root", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setup,
+	})
+}
+
+func setup(c *caddy.Controller) error {
+	config := dnsserver.GetConfig(c)
+
+	for c.Next() {
+		if !c.NextArg() {
+			return c.ArgErr()
+		}
+		config.Root = c.Val()
+	}
+
+	// Check if root path exists
+	_, err := os.Stat(config.Root)
+	if err != nil {
+		if os.IsNotExist(err) {
+			// Allow this, because the folder might appear later.
+			// But make sure the user knows!
+			log.Printf("[WARNING] Root path does not exist: %s", config.Root)
+		} else {
+			return c.Errf("Unable to access root path '%s': %v", config.Root, err)
+		}
+	}
+
+	return nil
+}