Add basic framework for writing a CLI plugin

That is, the helper to be used from the plugin's `main`.

Also add a `helloworld` plugin example and build integration.

Signed-off-by: Ian Campbell <ijc@docker.com>

cli-plugins/examples/helloworld/main.go

@@ -0,0 +1,38 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/docker/cli/cli-plugins/manager"
+	"github.com/docker/cli/cli-plugins/plugin"
+	"github.com/docker/cli/cli/command"
+	"github.com/spf13/cobra"
+)
+
+func main() {
+	plugin.Run(func(dockerCli command.Cli) *cobra.Command {
+		goodbye := &cobra.Command{
+			Use:   "goodbye",
+			Short: "Say Goodbye instead of Hello",
+			Run: func(cmd *cobra.Command, _ []string) {
+				fmt.Fprintln(dockerCli.Out(), "Goodbye World!")
+			},
+		}
+
+		cmd := &cobra.Command{
+			Use:   "helloworld",
+			Short: "A basic Hello World plugin for tests",
+			Run: func(cmd *cobra.Command, args []string) {
+				fmt.Fprintln(dockerCli.Out(), "Hello World!")
+			},
+		}
+
+		cmd.AddCommand(goodbye)
+		return cmd
+	},
+		manager.Metadata{
+			SchemaVersion: "0.1.0",
+			Vendor:        "Docker Inc.",
+			Version:       "0.1.0",
+		})
+}