Remember GDrive folder feature

This is a replica of a pull request I made earlier, but now it's purely based off the current state of roger/a-s-minus. For details on why I closed the earlier pull request, see this pull request on my own fork. Essentially, I made a Git blunder and had to fix it.

This is in response to #18. The feature adds a checkbox that, when checked, saves the currently selected GDrive folder (name and ID pair) to a localStorage key. When a screenshot is taken later, and the user opts to save it in GDrive, the saved folder is automatically loaded into the dropdown (instead of the default "My Drive"). When the selected folder is changed, the checkbox is unchecked, and the user must re-check the box to save the newly selected folder for next time.

Please take a look and consider merging this feature; I'll make any required style changes should there be any. Sorry for the headache of closing and opening multiple requests; still learning Git.

rojer / a-s-minus

Remember GDrive folder feature #25