support to save parsed cards info into a file

With this script and [disunity](https://github.com/ata4/disunity), we can update /db/cards.py very conveniently.

For example, in Mac,
```
./disunity.sh extract /Applications/Hearthstone/Data/OSX/cardxml0.unity3d

python3 cardxml.py /Applications/Hearthstone/Data/OSX/cardxml0/CAB-cardxml0/TextAsset/ ~/Desktop/cardinfo.out.txt
```

Then replace the variable '_cards' with the generated file.