feat: Add a new ErrorHandlerWithContext (#1328)

* feat: Add a new ErrorHandlerWithContext

This commit adds a new error handler, which is passed the 
current context, so that you can add custom redirects or even
other kinds of responses. For example:

```go
	e.Use(middleware.JWTWithConfig(middleware.JWTConfig{
		SigningKey: []byte("secret"),
		TokenLookup: "query:token",
		ErrorHandlerWithContext: func(err error, c echo.Context) error {
			// do stuff with context and err
			switch err.(type) {
			case jwt.ValidationError:
				return c.Redirect(http.StatusSeeOther, "/login")
			}
			return err
		},
	}))
```

* chore: address golint issues

middleware/jwt.go

@@ -25,6 +25,9 @@ type (
 		// ErrorHandler defines a function which is executed for an invalid token.
 		// It may be used to define a custom JWT error.
 		ErrorHandler JWTErrorHandler
+
+		// ErrorHandlerWithContext is almost identical to ErrorHandler, but it's passed the current context.
+		ErrorHandlerWithContext JWTErrorHandlerWithContext
 
 		// Signing key to validate token. Used as fallback if SigningKeys has length 0.
 		// Required. This or SigningKeys.
@@ -69,6 +72,9 @@ type (
 	// JWTErrorHandler defines a function which is executed for an invalid token.
 	JWTErrorHandler func(error) error
 
+	// JWTErrorHandlerWithContext is almost identical to JWTErrorHandler, but it's passed the current context.
+	JWTErrorHandlerWithContext func(error, echo.Context) error
+
 	jwtExtractor func(echo.Context) (string, error)
 )
 
@@ -177,6 +183,10 @@ func JWTWithConfig(config JWTConfig) echo.MiddlewareFunc {
 				if config.ErrorHandler != nil {
 					return config.ErrorHandler(err)
 				}
+
+				if config.ErrorHandlerWithContext != nil {
+					return config.ErrorHandlerWithContext(err, c)
+				}
 				return err
 			}
 			token := new(jwt.Token)
@@ -199,6 +209,9 @@ func JWTWithConfig(config JWTConfig) echo.MiddlewareFunc {
 			if config.ErrorHandler != nil {
 				return config.ErrorHandler(err)
 			}
+			if config.ErrorHandlerWithContext != nil {
+					return config.ErrorHandlerWithContext(err, c)
+			}
 			return &echo.HTTPError{
 				Code:     http.StatusUnauthorized,
 				Message:  "invalid or expired jwt",