add save_private_keys example

examples/save_private_keys.py

+from duniterpy.key import SigningKey
+import getpass
+
+# CONFIG #######################################
+
+# WARNING : Hide this file in a safe and secure place
+# If one day you forget your credentials,
+# you'll have to use one of your private keys instead
+PRIVATE_KEYS_FILE_PATH = "/home/vit/.duniter_account_private_keys.txt"
+
+################################################
+
+# prompt hidden user entry
+salt = getpass.getpass("Enter your passphrase (salt): ")
+
+# prompt hidden user entry
+password = getpass.getpass("Enter your password: ")
+
+# prompt public key
+pubkey = input("Enter your public key: ")
+
+# init signer instance
+signer = SigningKey(salt, password)\
+
+# check public key
+if signer.pubkey != pubkey:
+    print("Bad credentials !")
+    exit(0)
+
+# save private keys in a file (json format)
+signer.save(PRIVATE_KEYS_FILE_PATH)
+
+# document saved
+print("Private keys for public key %s saved in %s" % (pubkey, PRIVATE_KEYS_FILE_PATH))