[WIP] Kn plugins implementation

Implements Kn plugins re-using some code from kubectl plugins.
This is WIP to allow concrete discussions and testing and
hopefully also allow sub tasks to be created and worked on
in parallel.

This version contains the following:

1. wraps the main root Kn command to support plugin
2. plugins are any executable in PATH with name kn-*
3. 'kn plugins list' to list found kn plugins
4. skips any kn plugins found with name that match core
   commands, e.g., kn-service would be ignored
5. execute any valid kn plugins found
6. started adding UTs for plugin commands and files
7. rebased with latest HEAD

And is missing:

1. integration tests
2. plugin install command
3. plugin repository command
4. plugin / Knative server version negotiation
5. anything else we agree on in plugin req doc

go.mod

@@ -26,6 +26,8 @@ require (
 	github.com/mattbaird/jsonpatch v0.0.0-20171005235357-81af80346b1a // indirect
 	github.com/mitchellh/go-homedir v1.1.0
 	github.com/modern-go/reflect2 v1.0.1 // indirect
+	github.com/onsi/ginkgo v0.0.0-20170829012221-11459a886d9c
+	github.com/onsi/gomega v0.0.0-20170829124025-dcabb60a477c
 	github.com/peterbourgon/diskv v2.0.1+incompatible // indirect
 	github.com/spf13/cobra v0.0.3
 	github.com/spf13/pflag v1.0.3

pkg/kn/commands/plugin/plugin_suite_test.go

@@ -0,0 +1,27 @@
+// Copyright © 2018 The Knative Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package plugin_test
+
+import (
+	"testing"
+
+	. "github.com/onsi/ginkgo"
+	. "github.com/onsi/gomega"
+)
+
+func TestPlugin(t *testing.T) {
+	RegisterFailHandler(Fail)
+	RunSpecs(t, "Plugin Suite")
+}