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]
Add Rubocop to help maintain code style

Auto-generated due to amount of offenses detected, using Ruby 1.9.3 to include
encoding cop
Amend rubocop task to default rake task

Use simple and offenses formatters as this provides a details and a clear
summary of offenses after running
Ensure all strings use double quotes to follow style guide