vanished - set to true by a <vanish> action
rank - game difficulty, 0 to 1, default 0.5
tags - string tags set by the running actions
+ appearance - string used to set bullet appearance
Contructor Arguments:
- x, y, direction, speed, target, rank - same as the attributes
+ x, y, direction, speed, target, rank, tags, appearance
+ - same as the above attributes
actions - internal action list
Action - custom Action constructor