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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
```
options = super(VaultCLI, self).post_process_args(options)
        display.verbosity = options.verbosity
        if options.vault_ids:
            for vault_id in options.vault_ids:
                if u';' in vault_id:
                    raise AnsibleOptionsError("'%s' is not a valid vault id. The character ';' is not allowed in vault ids" % vault_id)
        if getattr(options, 'output_file', None) and len(options.args) > 1:
            raise AnsibleOptionsError("At most one input file may be used with the --output option")
        if options.action == 'encrypt_string':
            if '-' in options.args or not options.args or options.encrypt_string_stdin_name:
                self.encrypt_string_read_stdin = True
            
            if options.encrypt_string_prompt and self.encrypt_string_read_stdin:
                raise AnsibleOptionsError('The --prompt option is not supported if also reading input from stdin')
        return options
    def run(self):