New plugin "kn-plugin-search" for plugin discovery

In order to move forward with allowing users to discover available plugins, it is proposed to create a new plugin `kn-plugin-search` which introduces a `kn plugin search` command set queries a list of plugins from a fixed URL (e.g. hosted on the knative.dev website). This should be a YAML file, which contains information about:

* Plugin name
* Download location
* Version information
* Short description (1 line)
* Longer Description
* Usage Example
* Maybe: Tags

It should be possible to search with name and within the description. Also, an option should be added (e.g. `--all` for listing all plugins.

The output should by default fit on one line (like in `apt-cache search`), but a `--verbose` should enable the full output (including the full description & usage example). Maybe we could decide to show the full output, too, when only a single search result was detected.

This is the first step towards a more complete solution for dynamic discovery from within e.g. a GitHub organization via a naming convention, borrowing some ideas from `brew` and `krew`.