Allow `rain` to read from stdin - specifically for `rain diff`

[dothomson]

When reviewing template changes in a version control system the rain diff had proven to be very helpful to clearly state what will change from one deployment to the next.

However when the file being compared against is on a different git branch we have to write the file to disk before running rain diff against that template.

For example I will run the following commands:

git show main:./template.yaml > template-old.yaml
rain diff  template-old.yaml template.yaml

Taking inspiration from dyff having access to the stdin stream using - could simplify this process.

An example of what this could look like is:

git show main:./template.yaml | rain diff - template.yaml

Any way of achieving this would be a lovely additional feature. 🙇

[stilvoid]

I agree this would be a good feature and simple to implement. If I get some time, I'll submit a PR soon.

For now though, if you're on MacOS or Linux, you can use /dev/stdin for this purpose, e.g.:

git show main:./template.yaml | rain diff /dev/stdin template.yaml

I think the equivalent in Windows is conIN$ but I haven't tried it.

[stilvoid]

Feature implemented in #698