Replace duplicated `category` feature with `categories`
Add missed tests for categories
Add tests for duplicated categories
Mark certain gems as eventually-non-core.
Gracefully require
Put errors/exceptions into Jekyll::Errors module
Merge pull request #2591 from jekyll/extract-list
Update history to reflect merge of #2591 [ci skip]
Merge pull request #2584 from yous/patch-categories-feature
Update history to reflect merge of #2584 [ci skip]