Wrap flag usage to terminal size

[sixolet]

It's weird that the most idiomatic library to go get a term size is in a crypto lib, but it's not actually that bad of a dep and greatly improves the experience on a smaller-width terminal.

We don't bother to wrap if there's no gettable terminal size.

[rhuss]

Nice ;-) I wonder whether we should also add wrapping code for wide columns like the REASON in services list.

As you said, it's weird to include an ssh library for getting this functionality.
However, the alternatives like https://github.com/gdamore/tcell or https://github.com/nsf/termbox-go seems to be overkill for what we need here.

Some questions on dependencies, but otherwise fine.

[knative-metrics-robot]

The following is the coverage report on pkg/.
Say /test pull-knative-client-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/kn/core/root.go	59.3%	59.8%	0.5

[rhuss]

/lgtm
/approve

[knative-prow-robot]

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