Provide a -usage flag for printing the usage of a single plugin

Closes #136

config.go

@@ -421,3 +421,15 @@ func PrintSampleConfig() {
 		}
 	}
 }
+
+// PrintPluginConfig prints the config usage of a single plugin.
+func PrintPluginConfig(name string) error {
+	if creator, ok := plugins.Plugins[name]; ok {
+		plugin := creator()
+		fmt.Printf("# %s\n[%s]\n", plugin.Description(), name)
+		fmt.Printf(strings.TrimSpace(plugin.SampleConfig()))
+	} else {
+		return errors.New(fmt.Sprintf("Plugin %s not found", name))
+	}
+	return nil
+}