Allow none algorithm in jwt command

Usage:
```
$ echo '{"foo":"bar"}' | ./jwt -key /dev/null -alg none -sign - | ./jwt -key /dev/null -alg none -verify -
```

Signed-off-by: Alexander Yastrebov <yastrebov.alex@gmail.com>

cmd/jwt/main.go

@@ -138,6 +138,8 @@ func verifyToken() error {
 			return jwt.ParseRSAPublicKeyFromPEM(data)
 		} else if isEd() {
 			return jwt.ParseEdPublicKeyFromPEM(data)
+		} else if isNone() {
+			return jwt.UnsafeAllowNoneSignatureType, nil
 		}
 		return data, nil
 	})
@@ -240,6 +242,8 @@ func signToken() error {
 				return err
 			}
 		}
+	} else if isNone() {
+		key = jwt.UnsafeAllowNoneSignatureType
 	}
 
 	if out, err := token.SignedString(key); err == nil {
@@ -296,6 +300,10 @@ func isEd() bool {
 	return *flagAlg == "EdDSA"
 }
 
+func isNone() bool {
+	return *flagAlg == "none"
+}
+
 type ArgList map[string]string
 
 func (l ArgList) String() string {