Allow multiple extensions to handle the same event

[kylekatarnls]

Right now, an event listener option can pass only one event callback and if an extension use it, it erase previous callback possibly put by an other extension.

Tasks:

Allow array (multiple callbacks) arguments for all event options.

...
'on_format' => [$callback1, $callback2, $callback3],
...

Merge events from extensions into an array

class Extension1 extends \Phug\AbstractExtension
{
  public function getEvents()
  {
    return [
      'on_format' => [$c1, $c2],
      'on_parse' => $c3,
    ];
  }
}
class Extension2 extends \Phug\AbstractExtension
{
  public function getEvents()
  {
    return [
      'on_format' => $c4,
      'on_output' => [$c5],
    ];
  }
}
Phug::addExtension(Extension1::class);
Phug::addExtension(Extension2::class);

Phug::getOptions() should then contains:

[
  'on_format' => [$c1, $c2, $c4],
  'on_parse'  => [$c3],
  'on_output' => [$c5],
]

Documents new event option feature

https://phug-lang.com/#events

[kylekatarnls]

Available in 0.3.2