Add BotJoin Lua hook

Adds a Lua hook that enables the ability to modify a bot player as soon as it joins the game.

Hook format: addHook("PlayerJoin", functionname)

Function format: function(player_t player)

Executes each time a bot player is created or replaced—for example, when a GP starts, when a bot fills in a netgame spot, or when retired bots change character (note that this may occur outside of a level, such as during intermission).