Add support for selectors

[alikindsys]

Selectors can either be a player name, uuid or the @type[argument1=value,...] composite selector.
For the composite selector these arguments exist.
My proposed api :

val selector = NearestPlayerSelector {
    position {x = "~10", y = "~-10", z = "64"}
    gamemode { Gamemodes.CREATIVE } 
    team { "TeamName" }
}

That would be converted to the following selector
@p[x=~10,y=~-10,z=64,gamemode=creative,team=TeamName]

The equivalent for @a would be AllPlayersSelector