add static field to use as fixed "prefix"

[zonky2]

add static field at first to use as fixed "prefix" e.g. wie work with integer values like year or id...

in this time the alias add "id" if we use year 2017 to "id-2017-...." - after this we can use "year-2017-...."

[discordier]

Sounds reansonable, yet we should default it to id- to stay "compatible" with Contao

[zonky2]

e.g. from xtra

$strAlias = standardize(strip_tags($replaceEvent->getBuffer()));
if ('' !== $this->get('prefix')) {
    $strAlias = $this->get('prefix') . substr($strAlias, 3);
}

[discordier]

Remember to check for existence of 'id-' first!

[zonky2]

implemented in https://github.com/MetaModels/attribute_alias/pull/19