aliens runs a simulation of an alien attack.
aliens <input/file/path> N
where each line of the input file is of the format:
Foo north=Bar west=Baz south=Qu-ux
aliens creates a graph of the cities in the input, randomly assigns
aliens to cities. Aliens make random moves and destroy a city if two of them
are attacking the same city.
The output is a (pruned) graph of the remaining cities in the same format as the input.