feature: add gem-compare functionality (diffing versions)

[fwolfst]

Thanks. README states that feature ideas shall be created as tickets here.

My current workflow was bundle outdated # --filter-major and then copy-paste its output to gem compare. https://github.com/fedora-ruby/gem-compare

Also, we usually do it in batches (not all at once), in order to have separate CI runs and see green for each updated gem (or small group of gems).

[mattbrictson]

Thanks for the idea! I also have a similar workflow where I check the gem changelogs for major (and sometimes minor) version bumps.

Also, instead of gem-compare, I will use the Review changes link on the rubygems.org page (here's sidekiq 7.1.1 → 7.1.2, for example). Maybe bundleup could output those URLs so you can click from the console output directly into the diffs?