Add world map.

I added a world map in the game. Now player can input "look map" to see the it.
In the map, '.' is a tile you've discovered, '*' is the tile where the player is.

Example:
```
> look map
   .
   .
  ...*
   .
> go w
The path crosses a small stream.
There are some berries hanging from a tree. They look edible. If you're hungry enough, anyway.
> look map
    .
    .
   ..*.
    .
> 
```