oo7-substrate submit secret/private-key option

[retotrinkler]

Would be nice to have a secretKey alternative to the existing mnemonic phrase option something like the following:

	submitSecretKey (secretKey, name) {
		this._keys.push({secretKey, name})
		this._sync()
		return this.accountFromSecretKey(secretKey)
	}

	accountFromSecretKey (secretKey) {
		return new AccountId(nacl.sign.keyPair.fromSecretKey(secretKey).publicKey)
	}