Resolving #21 issue. Adding ko version and simple release script

[cezkuj]

Output from version command implemented as we have discussed in comments, easily we can extend it with other build information from govvv, if needed.
Also, added basic release script, I didn't have any guidelines so I added what seemed valid for me, let's discuss if it should be improved.
Cheers!

[cezkuj]

@jonjohnsonjr @imjasonh

I've updated branch so it doesn't have conflicts anymore. Could you review it?

[imjasonh]

I think we want to run git rev-parse in ko's directory in GOPATH, not in the current working directory, which is probably another repo and not ko's.

[cezkuj]

Hmm, agreed but, using GOPATH wouldn't be possible if migrated to go modules. It is definitely drawback - not sure if valid for now.

For the time being we could use git dir based on GOPATH. Smth like that:

gitDir := fmt.Sprintf("--git-dir=%v/src/github.com/google/ko/.git", os.Getenv("GOPATH"))
hash, err := exec.Command("git", gitDir, "rev-parse", "HEAD").Output()