Support a diff-strategy of "none" for "tk apply" to suppress diffing.

[jphx]

This pull request adds a diff strategy of none for tk apply that completely suppresses diffing before the apply operation.

The reason I'm submitting this is that I'm using Tanka to create a very large set of Kubernetes resources and it takes 5 to 10 minutes to perform the diff operation before getting to the apply stage. Since I often use a tk diff myself first, before the tk apply, I don't need the diff that's part of apply in some cases. Also, when our automated job runs to do a tk apply, it uses the --dangerous-auto-approve option, with no human present observe the diff output. The 5 to 10 minute delay in this step of the automation makes the automation job run longer than desired, delaying the deployment of the things that come after this step.

I would like to change some documentation to describe the none option to tk apply, but I don't see any good place to put that. Any suggestion would be welcome.

[jphx]

Would it be possible to get a review of this? It would be a useful enhancement for developers dealing with similar issues as myself.

[RealityAnomaly]

Would be great to have this in, I'm doing the same thing.

[julienduchesne]

LGTM!