cmd: list supported algorithms

The help message looks like:
```
-alg string
      signing algorithm identifier [ES256 ES384 ES512 EdDSA HS256 HS384 HS512 PS256 PS384 PS512 RS256 RS384 RS512 none]
```

Also fixes #63

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

cmd/jwt/main.go

@@ -14,14 +14,15 @@ import (
 	"io/ioutil"
 	"os"
 	"regexp"
+	"sort"
 	"strings"
 
 	"github.com/golang-jwt/jwt/v4"
 )
 
 var (
 	// Options
-	flagAlg     = flag.String("alg", "", "signing algorithm identifier")
+	flagAlg     = flag.String("alg", "", fmt.Sprintf("signing algorithm identifier %v", algs()))
 	flagKey     = flag.String("key", "", "path to key file or '-' to read from stdin")
 	flagCompact = flag.Bool("compact", false, "output compact JSON")
 	flagDebug   = flag.Bool("debug", false, "print out all kinds of debug data")
@@ -307,6 +308,12 @@ func isNone() bool {
 	return *flagAlg == "none"
 }
 
+func algs() []string {
+	algs := jwt.GetAlgorithms()
+	sort.Strings(algs)
+	return algs
+}
+
 type ArgList map[string]string
 
 func (l ArgList) String() string {

signing_method.go

@@ -33,3 +33,14 @@ func GetSigningMethod(alg string) (method SigningMethod) {
 	}
 	return
 }
+
+// GetAlgorithms returns a list of registered "alg" names
+func GetAlgorithms() (algs []string) {
+	signingMethodLock.RLock()
+	defer signingMethodLock.RUnlock()
+
+	for alg, _ := range signingMethods {
+		algs = append(algs, alg)
+	}
+	return
+}