kn plugins

[maximilien]

Add ability for the kn CLI to have plugins. This is particularly useful for three important scenarios:

1. allows users of kn to extend it with functionality
2. allows experimentation of commands that we don't want to include as a core command, e.g., $kn doctor
3. allows those wanting to package kn with their own offering of Knative to extend kn with additional commands that either:
   a. is not yet included in the current release
   b. are experimental
   c. are specific to a cloud environment, e.g., IBM's Cloud or Google Cloud

There could be additional reasons to have plugins to kn so please add them here. Also if you are against such a feature, please comment your reasoning here too.

[maximilien]

I will present and demo a proposal during Tuesday 04/06/2019's call.

[csantanapr]

I think this is good idea to have, and if it can be done by importing a package from kubectl source code to replicate the plugin system that kubectl has.

I would avoid clone and own code from kubectl.

This allows also for design and prototype how new commands and the output would like like.

[maximilien]

The following plugins WIP doc on my branch starts to motivate and give use cases to the proposed plugin that I will demo tomorrow.

[maximilien]

@sixolet has created a kn plugin requirements doc which will be used to implement this feature.

Please consider joining the discussion. Once stable I will summarize the agreement here and hopefully we can start to implement that feature.

[maximilien]

See PR #177 for first cut of plugins that match kubectl-style plugins.

[maximilien]

/assign @maximilien

[csantanapr]

This is great to see this feature landing
Thanks 💯 for the contribution @maximilien