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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
```
''' can encrypt any structured data file used by Ansible.
    This can include *group_vars/* or *host_vars/* inventory variables,
    variables loaded by *include_vars* or *vars_files*, or variable files
    passed on the ansible-playbook command line with *-e @file.yml* or *-e @file.json*.
    Role variables and defaults are also included!
    Because Ansible tasks, handlers, and other objects are data, these can also be encrypted with vault.
    If you'd like to not expose what variables you are using, you can keep an individual task file entirely encrypted.
    '''
    name = 'ansible-vault'
    FROM_STDIN = "stdin"
    FROM_ARGS = "the command line args"
    FROM_PROMPT = "the interactive prompt"
    def __init__(self, args):
        self.b_vault_pass = None
        self.b_new_vault_pass = None
        self.encrypt_string_read_stdin = False
        self.encrypt_secret = None
        self.encrypt_vault_id = None
        self.new_encrypt_secret = None
        self.new_encrypt_vault_id = None
        super(VaultCLI, self).__init__(args)
    def init_parser(self):