Add item interface

I try to add the item interface like the tile interface. If you want to create a new item, just create a module like `weatbag.items.sword` and then define the following methods in the module:

```python
def take(tile, player):
    # Called when player try to take the item.
    ...

def use(tile, player):
    # Called when player try to use the item.
    ...

def drop(tile, player):
    # Called when player try to drop the item.
    ...
```
Then, when player try to pick or use the item in the game, just do:

```python
from weatbag.items import take
...
class Tile:
    ...
    def action(self, player, do):
        ...
        if do[0] in words.take:
            take(do[1], self, player)
        ...
```
I rewrote the code about berries as an example.
If you guys have any better ideas please tell me, I am happy to know.