add command to convert pubkey to ipv6 addr

standalone cli tool like genkeys/main.go that uses
the address.AddrForKey function to convert a pubkey
to an IPv6 address.

```
+$ go run main.go <thepubkey>.
+IP: <theipv6ip>
```

cmd/addrforkey/main.go

@@ -0,0 +1,26 @@
+/*
+
+This file converts a public to to an IPv6 address. Example:
+$ go run main.go <pubkey>.
+IP: <ipv6ip>
+
+*/
+package main
+
+import (
+	"encoding/hex"
+	"fmt"
+	"net"
+	"os"
+	"github.com/yggdrasil-network/yggdrasil-go/src/address"
+)
+
+func main() {
+        pub, err := hex.DecodeString(os.Args[1])
+	if err != nil {
+                fmt.Println("Failed to decode provided public key")
+                os.Exit(1)
+	}
+        addr := address.AddrForKey(pub)
+        fmt.Println("IP:", net.IP(addr[:]).String())
+}