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##### SUMMARY
<!--- Describe the new feature/improvement briefly below -->
Whenever I use `ansible-vault encrypt_string`, I want to take the result and add it to a variable file, however I have to manually select the text/copy/open file for editing/paste/save the change.  This process is painful especially when you have multiple strings to encrypt as it doesn't work with the Unix way of things thus being very manual.   I thought `ansible-vault encrypt_string --output` might be something to help here, but the help documentation is confusing as it appears this is only used with `encrypt` and `decrypt` _(commands)_.

##### ISSUE TYPE
- Feature Idea

##### COMPONENT NAME
<!--- Write the short name of the module, plugin, task or feature below, use your best guess if unsure -->
lib/ansible/cli/vault.py

##### ADDITIONAL INFORMATION
<!--- Describe how the feature would be used, why it is needed and what it would solve -->
The only way I can see this working _without coming up with custom file descriptors or repurposing stderr for encrypted output so redirection will work_ is to add a new option that could append the resulting string to a file.
```shell
ansible-vault encrypt_string --encrypt-vault-id default --stdin-name --append vars_file.yml
Reading plaintext input from stdin. (ctrl-d to end input)
hunter22
Encryption successful
```
edit_parser.add_argument('args', help='Filename', metavar='file_name', nargs='*')
        view_parser = subparsers.add_parser('view', help='View vault encrypted file', parents=[common])
        view_parser.set_defaults(func=self.execute_view)
        view_parser.add_argument('args', help='Filename', metavar='file_name', nargs='*')
        encrypt_parser = subparsers.add_parser('encrypt', help='Encrypt YAML file', parents=[common, output, vault_id])
        encrypt_parser.set_defaults(func=self.execute_encrypt)
        encrypt_parser.add_argument('args', help='Filename', metavar='file_name', nargs='*')
        enc_str_parser = subparsers.add_parser('encrypt_string', help='Encrypt a string', parents=[common, output, vault_id])
        enc_str_parser.set_defaults(func=self.execute_encrypt_string)
        enc_str_parser.add_argument('args', help='String to encrypt', metavar='string_to_encrypt', nargs='*')
        enc_str_parser.add_argument('-p', '--prompt', dest='encrypt_string_prompt',
                                    action='store_true',
                                    help="Prompt for the string to encrypt")
        enc_str_parser.add_argument('--show-input', dest='show_string_input', default=False, action='store_true',
                                    help='Do not hide input when prompted for the string to encrypt')
        enc_str_parser.add_argument('-n', '--name', dest='encrypt_string_names',
                                    action='append',
                                    help="Specify the variable name")
        enc_str_parser.add_argument('--stdin-name', dest='encrypt_string_stdin_name',
                                    default=None,
                                    help="Specify the variable name for stdin")
        rekey_parser = subparsers.add_parser('rekey', help='Re-key a vault encrypted file', parents=[common, vault_id])
        rekey_parser.set_defaults(func=self.execute_rekey)
        rekey_new_group = rekey_parser.add_mutually_exclusive_group()
        rekey_new_group.add_argument('--new-vault-password-file', default=None, dest='new_vault_password_file',
                                     help="new vault password file for rekey", type=opt_help.unfrack_path())
        rekey_new_group.add_argument('--new-vault-id', default=None, dest='new_vault_id', type=str,
                                     help='the new vault identity to use for rekey')
        rekey_parser.add_argument('args', help='Filename', metavar='file_name', nargs='*')
    def post_process_args(self, options):