Add flag for specifying secret key

[networkException]

This pull request adds a command line flag allowing users to pass a hex encoded ed25519 private key to be used for the router identity keypair.

Signed-off-by: networkException <git@nwex.de> (private sign-off)

[neilalexander]

This isn’t using the hex-provided input as the private key, this is using the hex-provided input as a seed to the random generator. I assume this isn’t what you intended?

[networkException]

The implementation is just using the seed as the private key, I'm not sure if thats a great idea but...

Here its reading the seed from the passed reader to then generate a new keypair which just gets the seed set as the private key

[networkException]

Do you know of a better way to do this?

[devonh]

Instead of using GenerateKey when a private key is provided, could you just use NewKeyFromSeed directly?
It's a little clearer to understand since there is no indirection through the rand reader.
ie. something like:

	var sk ed25519.PrivateKey
	if len(*secretkey) != 0 {
		secretkeyHex, err := hex.DecodeString(*secretkey)
		if err != nil {
			panic(err)
		}

		sk = ed25519.NewKeyFromSeed(secretkeyHex)
	} else {
		var err error
		_, sk, err = ed25519.GenerateKey(nil)
		if err != nil {
			panic(err)
		}
	}