Where to programmatically enable a Feature module
A feature module is just another module. And you can enable it programmatically using the linked code from another enabled module's hook_update_N placed in this another enabled module's *.install file. This even can be another feature module. Then have this hook fired via drush updb on the command line or by visiting your site's /update.php.

You also could do that from a form submit handler. Or even from a menu callback providing some custom endpoint you can access with cURL for example.

It's safe to use other feature modules' *.install files. Since Features never touches *.module and *.install files during regeneration. So you are free to use them to your needs.