Add command for tagging revisions

[navidshaikh]

This is followup issue of #231, the idea is to introduce leaf command for labeling revisions.
Couple of verb choices: tag, label
I personally like kn revision tag.

[navidshaikh]

/assign @navidshaikh

[rhuss]

I definitley all for tag to avoid any confustion with metadata.labels

[maximilien]

Yup. We had that in knctl. Should be also useful for how we end up doing #210.

[sixolet]

Fixed by #345