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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
```
''' encrypt the supplied file using the provided vault secret '''
        if not context.CLIARGS['args'] and sys.stdin.isatty():
            display.display("Reading plaintext input from stdin", stderr=True)
        for f in context.CLIARGS['args'] or ['-']:
            
            self.editor.encrypt_file(f, self.encrypt_secret,
                                     vault_id=self.encrypt_vault_id,
                                     output_file=context.CLIARGS['output_file'])
        if sys.stdout.isatty():
            display.display("Encryption successful", stderr=True)
    @staticmethod
    def format_ciphertext_yaml(b_ciphertext, indent=None, name=None):
        indent = indent or 10
        block_format_var_name = ""
        if name:
            block_format_var_name = "%s: " % name
        block_format_header = "%s!vault |" % block_format_var_name
        lines = []
        vault_ciphertext = to_text(b_ciphertext)
        lines.append(block_format_header)
        for line in vault_ciphertext.splitlines():
            lines.append('%s%s' % (' ' * indent, line))
        yaml_ciphertext = '\n'.join(lines)
        return yaml_ciphertext
    def execute_encrypt_string(self):