Add a "version" command

[uablrek]

Adds;

kpng version

The main.version string is defined at compile time using -X main.version=(something). Example;

CGO_ENABLED=0 GOOS=linux go build -ldflags "-extldflags '-static' -X main.version=$(git rev-parse --short HEAD)" -o kpng ./cmd/kpng

Now the git commit is printed;

$ ./kpng version
95df644

When developing when the commit is not updated a date may be more useful;

CGO_ENABLED=0 GOOS=linux go build -ldflags "-extldflags '-static' -X main.version=$(date +%F:%T)" -o kpng ./cmd/kpng
./kpng version
2021-05-11:11:20:36

NOTE; I have deliberately made the printout clean. No "Kpng version:" prefix or something. The version will most likely be interpreted by some script or test-report tool and unnecessary decorations are just annoying. And humans understand it just as good.

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[uablrek]

And IMO the "version" command and variable belongs in main.go. That's where you would expect to find it. To have it in a separate file or dir would be to over-structure things.

[mcluseau]

thanks :)

/lgtm
/approve

[uablrek]

@mcluseau PTAL. I had to amend the commit message.
The commit is the same.

[mcluseau]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mcluseau, uablrek

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