Add support for --scale-init

[duglin]

Now that we have support for defining the initial scale for a service, it would be nice if kn supported it via a top-level option instead of just as an annotation.

e.g.
kn service create echo --image duglin/echo --annotation autoscaling.knative.dev/initialScale=0
vs
kn service create echo --image duglin/echo --scale-init=0

or --scale-initial

[hemanrnjn]

/assign