This allows the following invocations:

    # Interactive use, like gpg
    ansible-vault encrypt --output x

    # Non-interactive, for scripting
    echo plaintext|ansible-vault encrypt --output x

    # Separate input and output files
    ansible-vault encrypt input.yml --output output.yml

    # Existing usage (in-place encryption) unchanged
    ansible-vault encrypt inout.yml

…and the analogous cases for ansible-vault decrypt as well.

In all cases, the input and output files can be '-' to read from stdin
or write to stdout. This permits sensitive data to be encrypted and
decrypted without ever hitting disk.