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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
```
raise AnsibleOptionsError("A vault password is required to use Ansible's Vault")
        if action in ['encrypt', 'encrypt_string', 'create']:
            encrypt_vault_id = None
            
            if action not in ['edit']:
                encrypt_vault_id = context.CLIARGS['encrypt_vault_id'] or C.DEFAULT_VAULT_ENCRYPT_IDENTITY
            vault_secrets = None
            vault_secrets = \
                self.setup_vault_secrets(loader,
                                         vault_ids=vault_ids,
                                         vault_password_files=list(context.CLIARGS['vault_password_files']),
                                         ask_vault_pass=context.CLIARGS['ask_vault_pass'],
                                         create_new_password=True)
            if len(vault_secrets) > 1 and not encrypt_vault_id:
                raise AnsibleOptionsError("The vault-ids %s are available to encrypt. Specify the vault-id to encrypt with --encrypt-vault-id" %
                                          ','.join([x[0] for x in vault_secrets]))
            if not vault_secrets:
                raise AnsibleOptionsError("A vault password is required to use Ansible's Vault")
            encrypt_secret = match_encrypt_secret(vault_secrets,
                                                  encrypt_vault_id=encrypt_vault_id)
            
            
            self.encrypt_vault_id = encrypt_secret[0]
            self.encrypt_secret = encrypt_secret[1]
        if action in ['rekey']:
            encrypt_vault_id = context.CLIARGS['encrypt_vault_id'] or C.DEFAULT_VAULT_ENCRYPT_IDENTITY