Refactor and add support for inpainting

[charlesjlee]

This is a big PR. It's too big and I know it, but I'm too lazy to break it up into smaller pieces. I'm happy to go through multiple revisions here. The three main changes are:

1. Extracting common logic into their own methods
2. Adding support for the inpainting API
3. Expanding examples in the README

I broke some things, notably:

• I used f-string's, which require Python 3.6+
• I removed a special batch generation tracking log that was printed by generate_amount()
• I changed the image download type from jpg to png

[ezzcodeezzlife]

Thanks for the PR and all the feature ideas in the new issues. Really improves this package a lot ❤️