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/config/configfile/file.go

@@ -49,6 +49,7 @@ type ConfigFile struct {
 	StackOrchestrator    string                      `json:"stackOrchestrator,omitempty"`
 	Kubernetes           *KubernetesConfig           `json:"kubernetes,omitempty"`
 	CurrentContext       string                      `json:"currentContext,omitempty"`
+	CLIPluginsExtraDirs  []string                    `json:"cliPluginsExtraDirs,omitempty"`
 }
 
 // ProxyConfig contains proxy configuration settings