Support machine-readable output for "kn version"

[rhuss]

In what area(s)?

Describe the feature:

In order to interoperate nicely with tooling that calls kn and in the spirit of our other commands we should support machine-readable output in addition to human-readable output for kn version.

I.e. I think having a kn version -o json return something like

{
  "version": "v0.10.0",
  "date": "2019-11-21 21:06:36",
  "commit": "e07b5a9",
  "apis": {
     "serving": [
        "serving.knative.dev/v1alpha1"
     ],
     "eventing": [
        "sources.eventing.knative.dev/v1alpha1"
     ]
  } 
}

In the future there might be server version (i.e. which APIs are supported), included here as well.

[zhanggbj]

/assign

[navidshaikh]

@zhanggbj are you working on this?

[navidshaikh]

/assign