Align the --to param to comply with standard kn --sink param

[cardil]

During the development of the kn event plugin, two options for sending have been implemented: --to and --to-url. Both of them are being described in this doc, in appropriate sections.

It makes sense to align both of them in single --to option, as the sink command line spec format is way easier to write than the current subject-based one. Maybe, to keep the functionality of matching resources by labels and targeting resources besides Knative, but addressable, appropriate contributions can be made to knative client itself to extend the sink spec format to accept subject-style as fallback.

Also, the --to will accept http(s) addresses.

/kind enhancement

[cardil]

Splits from #222

[cardil]

/triage accepted

[cardil]

It was decided that folding client-pkg back to the client should be done: knative/client#1941

The client-pkg code is needed to complete this issue, so knative/client#1941 effectively blocks this.

[cardil]

/assign