Feature request: DijkstraMap find_lowest_exits function

In a 4-way movement roguelike, there are cases where there can be two movement directions that cost the same amount in the ```DijkstraMap```. This is a request to add a function to return a vector of the lowest exit indices for further processing.

You can see the issue this causes for monster movement with only having ```find_lowest_exit``` in HandsOnRust: they favor moving in dog legs chasing the player east/west and will never try moving north/south towards the player if the player moves east/west and is aligned with the monster.