feat(scaffold): add support for variables

[kate-goldenring]

fixes #31

Try it out

1. Set up a SpinKube cluster
2. Apply an example app, supplying that required foo and bar variables using the new flag

go run main.go scaffold --from http://ghcr.io/kate-goldenring/spin-foo-bar-variables:v1 --variable foo=yoo --variable bar=yee --replicas 1 | kubectl apply -f -

3. Port forward the k8s svc and curl the endpoint to see the variable values logged

[bacongobbler]

Thanks @kate-goldenring! Would you mind adding a test under scaffold_test.go to make sure the indentation looks correct to you?

[kate-goldenring]

@bacongobbler added

[kate-goldenring]

@bacongobbler this should be ready for another review