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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
```
super(VaultCLI, self).init_parser(
            desc="encryption/decryption utility for Ansible data files",
            epilog="\nSee '%s <command> --help' for more information on a specific command.\n\n" % os.path.basename(sys.argv[0])
        )
        common = opt_help.argparse.ArgumentParser(add_help=False)
        opt_help.add_vault_options(common)
        opt_help.add_verbosity_options(common)
        subparsers = self.parser.add_subparsers(dest='action')
        subparsers.required = True
        output = opt_help.argparse.ArgumentParser(add_help=False)
        output.add_argument('--output', default=None, dest='output_file',
                            help='output file name for encrypt or decrypt; use - for stdout',
                            type=opt_help.unfrack_path())
        
        vault_id = opt_help.argparse.ArgumentParser(add_help=False)
        vault_id.add_argument('--encrypt-vault-id', default=[], dest='encrypt_vault_id',
                              action='store', type=str,
                              help='the vault id used to encrypt (required if more than one vault-id is provided)')
        create_parser = subparsers.add_parser('create', help='Create new vault encrypted file', parents=[vault_id, common])
        create_parser.set_defaults(func=self.execute_create)
        create_parser.add_argument('args', help='Filename', metavar='file_name', nargs='*')
        decrypt_parser = subparsers.add_parser('decrypt', help='Decrypt vault encrypted file', parents=[output, common])
        decrypt_parser.set_defaults(func=self.execute_decrypt)
        decrypt_parser.add_argument('args', help='Filename', metavar='file_name', nargs='*')
        edit_parser = subparsers.add_parser('edit', help='Edit vault encrypted file', parents=[vault_id, common])
        edit_parser.set_defaults(func=self.execute_edit)