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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
```
from __future__ import (absolute_import, division, print_function)
__metaclass__ = type
from ansible.cli import CLI
import os
import sys
from ansible import constants as C
from ansible import context
from ansible.cli.arguments import option_helpers as opt_help
from ansible.errors import AnsibleOptionsError
from ansible.module_utils._text import to_text, to_bytes
from ansible.parsing.dataloader import DataLoader
from ansible.parsing.vault import VaultEditor, VaultLib, match_encrypt_secret
from ansible.utils.display import Display
display = Display()
class VaultCLI(CLI):