MBTiles format control (jpg or png)

Hello. I added some lines to control MBTiles images format (jpg or png). If you consider it right you can merge my pull request. In addition, since I wanted to make calls to functionality from other utilities inside a Python environment (not from command line), and keep clean your original project, I created a new project for that. I mention you as the original author of the command line utility:
https://github.com/cayetanobv/MBTiles-extractor-lib
Comments, changes or suggestions are welcome.
Thanks for all!