Add support for running a CLI plugin

Also includes the  scaffolding for finding a validating plugin candidates.

Argument validation is moved to RunE to support this, so `noArgs` is removed.

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

cli-plugins/examples/helloworld/main.go

@@ -33,6 +33,6 @@ func main() {
 		manager.Metadata{
 			SchemaVersion: "0.1.0",
 			Vendor:        "Docker Inc.",
-			Version:       "0.1.0",
+			Version:       "testing",
 		})
 }