Make image key configurable for thumbnails/images

[Schallbert]

Why this change?

Some people like me use themes where the image key in a post's front matter is either already taken (e.g. for a banner image) or unused.

Some themes like the one I use (minimal mistakes) has a different way of organizing a post's frontmatter like header.teaser.

This PR should solve this issue with a customizable key: thumb_path_key can be set in _config.yml and will point the liquid template to the correct thumbnail image path.

This can help deduplicate front matter information.

What else I touched

- I have reorganized the generator's code just a bit so its main routine looks shorter for readability.

• In the liquid template, thumb_path_key is evaluated and split to allow for higher nesting depths in the front matter like header.teaser
• I updated the affected readme section with examples, might be a bit lengthy now.

Still this change keeps backwards compatibility due to default values for image and image.path.

Testing

• add tests for changes in feed.xml
• add tests for the updated .rb paths

[parkr]

Thanks for the PR! It's generally good hygiene to keep each PR narrowly focused. Would you mind pulling your refactor into a separate PR? Anything directly related to the image path change is fine to stay.

[Schallbert]

OK, will do.
I'll also write some tests for the new behavior.

[parkr]

Thank you!

[Schallbert]

I hope we should be good to go now.