Allow read data from stdin on Windows

[jkroepke]

Fixes #739

Allow - as cross platform alternative to /dev/stdin for reading data from STDIN.

% cat secrets.yaml | ~/go/src/go.mozilla.org/sops/cmd/sops/sops --input-type yaml --output-type yaml -d -
podAnnotations:
    secret: value
% ~/go/src/go.mozilla.org/sops/cmd/sops/sops -d secrets.yaml
podAnnotations:
    secret: value

[jkroepke]

In case tests are missing, please give me a hint where should I add the tests, thanks.

[jkroepke]

How to move forward here?

[jkroepke]

@ajvb (chosen as an active maintainer) could you take a look here?

[iverberk]

@ajvb any change of taking a look and merging? This is a nice fix that resolves having to use a custom build of SOPS on Windows

[jkroepke]

Changing merge target to develop

[jkroepke]

Reading https://github.com/mozilla/sops/discussions/927, its like it will take some time.