Support pkcs8 from Apple APNs

[conotto]

Hi,
After spending a few hours on trying to import Apple Mapkit p8 file, which is pkcs8, i have stumbled on someones post here:
dgrijalva/jwt-go#179

It includes a working function to import the private key, i've tested it and it works.
While i am not the author of the post in the link, i feel it would be beneficial to many people to have this function be included in your repo.
Would you be interested in a pull request with the above mentioned change ?
I will make sure to include the original author.

[oxisto]

Isn't this solved by this function here?

jwt/ecdsa_utils.go

Lines 15 to 40 in b88a60f

	// ParseECPrivateKeyFromPEM parses a PEM encoded Elliptic Curve Private Key Structure
	func ParseECPrivateKeyFromPEM(key []byte) (*ecdsa.PrivateKey, error) {
	var err error
	// Parse PEM block
	var block *pem.Block
	if block, _ = pem.Decode(key); block == nil {
	return nil, ErrKeyMustBePEMEncoded
	}
	// Parse the key
	var parsedKey interface{}
	if parsedKey, err = x509.ParseECPrivateKey(block.Bytes); err != nil {
	if parsedKey, err = x509.ParsePKCS8PrivateKey(block.Bytes); err != nil {
	return nil, err
	}
	}
	var pkey *ecdsa.PrivateKey
	var ok bool
	if pkey, ok = parsedKey.(*ecdsa.PrivateKey); !ok {
	return nil, ErrNotECPrivateKey
	}
	return pkey, nil
	}