Add -version flag for carbonapi executable

[martialblog]

Hi,

whilst testing carbonapi I was noticing there was no version flag. This PR implements this flag similar to go-carbon.

./carbonapi -version
0.17.0
v0.17.0-23-gfc33-dirty

I think it would also make sense to add more context to the output, meaning what the two lines mean:

./carbonapi -version
Version: 0.17.0
BuildVersion: v0.17.0-23-gfc33-dirty

But, I went for consistency with go-carbon. Alternatively, I could add the suggestion here and also open a PR in the go-carbon repo.

Regards,
Markus