Accept a config file path instead of a config name

Config name is a Viper concept used for searching a specific file
in various paths with various extensions.

Making it configurable is usually not a useful feature
as users mostly want to define a full or relative path
to a config file.

This change replaces config name with config file.

cmd/install_windows.go

@@ -64,7 +64,7 @@ func getCustomServeFlags() []string {
 		result = append(result, "--"+configDirFlag)
 		result = append(result, configDir)
 	}
-	if configFile != defaultConfigName {
+	if configFile != "" {
 		result = append(result, "--"+configFileFlag)
 		result = append(result, configFile)
 	}