Add a .hidden attribute to achievement prototypes

[eradicator]

What?

Allow hiding certain achivements by specifying the prototype as .hidden = true. A hidden achivement should only be shown in the menu after it has been unlocked/"achieved" by the player.

Why?

Achivements with hidden conditions are already in the game. I'd like to go one step further and not reveal that the achivement exists at all until the player does get it. Because i'd like some achivements that the player can find serendipitously without being frustrated and googling for it instead (like the dolphin one for strags).