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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
```
if hide_input:
                msg = "String to encrypt (hidden): "
            else:
                msg = "String to encrypt:"
            prompt_response = display.prompt(msg, private=hide_input)
            if prompt_response == '':
                raise AnsibleOptionsError('The plaintext provided from the prompt was empty, not encrypting')
            b_plaintext = to_bytes(prompt_response)
            b_plaintext_list.append((b_plaintext, self.FROM_PROMPT, name))
        
        if self.encrypt_string_read_stdin:
            if sys.stdout.isatty():
                display.display("Reading plaintext input from stdin. (ctrl-d to end input, twice if your content does not already have a newline)", stderr=True)
            stdin_text = sys.stdin.read()
            if stdin_text == '':
                raise AnsibleOptionsError('stdin was empty, not encrypting')
            if sys.stdout.isatty() and not stdin_text.endswith("\n"):
                display.display("\n")
            b_plaintext = to_bytes(stdin_text)
            
            name = context.CLIARGS['encrypt_string_stdin_name']
            b_plaintext_list.append((b_plaintext, self.FROM_STDIN, name))
        
        
        if context.CLIARGS.get('encrypt_string_names', False):
            name_and_text_list = list(zip(context.CLIARGS['encrypt_string_names'], args))
            
            if len(args) > len(name_and_text_list):
                
                display.display('The number of --name options do not match the number of args.',