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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
```
show_delimiter = False
        if len(b_plaintext_list) > 1:
            show_delimiter = True
        
        output = []
        
        
        
        for index, b_plaintext_info in enumerate(b_plaintext_list):
            
            b_plaintext, src, name = b_plaintext_info
            b_ciphertext = self.editor.encrypt_bytes(b_plaintext, self.encrypt_secret,
                                                     vault_id=vault_id)
            
            yaml_text = self.format_ciphertext_yaml(b_ciphertext, name=name)
            err_msg = None
            if show_delimiter:
                human_index = index + 1
                if name:
                    err_msg = '# The encrypted version of variable ("%s", the string #%d from %s).\n' % (name, human_index, src)
                else:
                    err_msg = '# The encrypted version of the string #%d from %s.)\n' % (human_index, src)
            output.append({'out': yaml_text, 'err': err_msg})
        return output
    def execute_decrypt(self):