Support true plugins

[hanneskod]

While working with https://github.com/hanneskod/giroapp-mailer-plugin it became clear we need a better plugin system.

• Plugins need to be able to load classes from filesystem.
• Plugins need to load settings.
• Plugins may want to add commands and other stuffs. Not merely event listeners..

1. Proposition to create a PluginInterface something like:

interface PluginInterface
{
    public function loadPlugin($settings); // called with a giroapp settings object
    public function getSubscribers(): SubscriberInterface[];
    public function getCommands(): CommandInterface[];
    // ...
}

2. Then let plugins be a file in /plugins that return a PluginInterface object. Be it a phar archive or a regular php file.. So the loading mechanism should be something like:

$plugin = require 'plugins/my-cool-plugin.phar';
$plugin->loadPlugin($giroappSettings);
// ...

3. And lastly let the plugin keep settings that should or could be tweeked by the user in the regular settings repository.