Avoid error on a missing plugin

On adding a new plugin in the configuration, if the related file isn't include, the lib fails to start.

Instead, it displays a `console.log()` message indicating the missing plugin and does not kill the plugin. The lib will start without this plugin activated.

It can also fire an alert message but I prefer a silent `console.log()`. If this case happen, the developper will (of course) look into the console and will see the message. Tell me if you prefer an alert instead of a `console.log()` (or maybe a custom exception ?).

I've added a test to reproduce the bug.