Include tag in resolved references

[jonjohnsonjr]

If a single tag is explicitly set (i.e. it's not "latest"), include that
in the reference that gets rendered in the yaml.

This is really useful for tracking releases.

Fixes #108

Also fixes a TODO.

[jonjohnsonjr]

One tag:

$ ko publish -B -t hello ./cmd/ko
2019/11/07 13:45:38 Using base gcr.io/distroless/static:latest for github.com/google/ko/cmd/ko
2019/11/07 13:45:38 Building github.com/google/ko/cmd/ko
2019/11/07 13:45:41 Publishing gcr.io/jonjohnson-test/ko/ko:hello
2019/11/07 13:45:42 Published gcr.io/jonjohnson-test/ko/ko:hello@sha256:545110ba5bbd8290575ec034b2caf49de8f665d41368bef6f4a955878780944f
gcr.io/jonjohnson-test/ko/ko:hello@sha256:545110ba5bbd8290575ec034b2caf49de8f665d41368bef6f4a955878780944f

Two tags:

$ ko publish -B -t hello -t world ./cmd/ko
2019/11/07 13:45:56 Using base gcr.io/distroless/static:latest for github.com/google/ko/cmd/ko
2019/11/07 13:45:56 Building github.com/google/ko/cmd/ko
2019/11/07 13:46:00 Publishing gcr.io/jonjohnson-test/ko/ko:hello
2019/11/07 13:46:01 Tagging gcr.io/jonjohnson-test/ko/ko:world
2019/11/07 13:46:02 Published gcr.io/jonjohnson-test/ko/ko@sha256:545110ba5bbd8290575ec034b2caf49de8f665d41368bef6f4a955878780944f
gcr.io/jonjohnson-test/ko/ko@sha256:545110ba5bbd8290575ec034b2caf49de8f665d41368bef6f4a955878780944f

[vdemeester]

LGTM 😻

[afrittoli]

Nice, thank you!