Auto adds default placeholders for GameModeAddons

[tastybento]

#322

[Poslovitch]

Has been agreed on Discord that proceeding in the following manner should be investigated:

enum Placeholders {
    ISLAND_MEMBERS_COUNT("island-members-count", (island) -> island.getMembers().size());

    public Placeholders(String placeholder, Consumer placeholderReplacer) {...}

    public String getPlaceholder() {...}
    public Consumer getReplacer() {...}
}

[Poslovitch]

Okay, so... I couldn't come up with a better design, actually.
This PR has a very drafty feeling. But I'll merge it. We'll see how it goes.

Couldn't come up with a better design