`-m --mask` option for `save_gdal/qgis.py`

[falkamelung]

Description of the desired feature

Our system generates a couple of different mask files and a save_gdal.py --mask option will make it easier to use and keep track of them.

I am happy to submit a PR on this when I get a chance.

Are you willing to help implement and maintain this feature?

• [ X] Yes
• [ ] No

[yunjunz]

Sure, please feel free to submit a PR when you get a chance. You could use save_kmz.py --mask option as a reference.